Bug: do not update `perform_session_flow_routing` output if the `SessionRoutingChoice` is none



While performing the session token routing we create an empty vec that will contain the Routing choices for that particular session. In case if there is no connectors that requires session call then the `perform_session_flow_routing` will return the empty vec. Instead if there is no connectors that requires session call, then we should just return empty hashmap.
diff --git a/crates/router/src/core/payments/routing.rs b/crates/router/src/core/payments/routing.rs
index d1a5441d958..d6904e09a3d 100644
--- a/crates/router/src/core/payments/routing.rs
+++ b/crates/router/src/core/payments/routing.rs
@@ -1009,7 +1009,9 @@ pub async fn perform_session_flow_routing(
                     });
                 }
             }
-            result.insert(pm_type, session_routing_choice);
+            if !session_routing_choice.is_empty() {
+                result.insert(pm_type, session_routing_choice);
+            }
         }
     }
 

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
While performing the session token routing we create an empty vec that will contain the Routing choices for that particular session. In case if there is no connectors that requires session call then the `perform_session_flow_routing` will return the empty vec. Instead if there is no connectors that requires session call, then we should just return empty hashmap.

https://github.com/juspay/hyperswitch/pull/5336

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
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## Motivation and Context
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If it fixes an open issue, please link to the issue here.

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## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
-> Create mca for stripe with apple pay, google pay and klarna
```
{
    "connector_type": "fiz_operations",
    "connector_name": "stripe",
    "connector_account_details": {
        "auth_type": "HeaderKey",
        "api_key": ""
    },
    "test_mode": true,
    "disabled": false,
    "payment_methods_enabled": [
        {
            "payment_method": "card",
            "payment_method_types": [
                {
                    "payment_method_type": "credit",
                    "payment_experience": null,
                    "card_networks": [
                        "Visa",
                        "Mastercard"
                    ],
                    "accepted_currencies": null,
                    "accepted_countries": null,
                    "minimum_amount": -1,
                    "maximum_amount": 68607706,
                    "recurring_enabled": true,
                    "installment_payment_enabled": true
                },
                {
                    "payment_method_type": "debit",
                    "payment_experience": null,
                    "card_networks": [
                        "Visa",
                        "Mastercard"
                    ],
                    "accepted_currencies": null,
                    "accepted_countries": null,
                    "minimum_amount": -1,
                    "maximum_amount": 68607706,
                    "recurring_enabled": true,
                    "installment_payment_enabled": true
                }
            ]
        },
        {
            "payment_method": "wallet",
            "payment_method_types": [
                {
                    "payment_method_type": "apple_pay",
                    "payment_experience": "invoke_sdk_client",
                    "minimum_amount": 1,
                    "maximum_amount": 68607706,
                    "recurring_enabled": true,
                    "installment_payment_enabled": true
                },
                {
                    "payment_method_type": "google_pay",
                    "payment_experience": "invoke_sdk_client",
                    "minimum_amount": 1,
                    "maximum_amount": 68607706,
                    "recurring_enabled": true,
                    "installment_payment_enabled": true
                }
            ]
        },
        {
            "payment_method": "pay_later",
            "payment_method_types": [
                {
                    "payment_method_type": "klarna",
                    "payment_experience": "redirect_to_url",
                    "card_networks": null,
                    "accepted_currencies": null,
                    "accepted_countries": null,
                    "minimum_amount": 1,
                    "maximum_amount": 68607706,
                    "recurring_enabled": true,
                    "installment_payment_enabled": true
                }
            ]
        }
    ],
    "metadata": {
        "apple_pay_combined": {
            "manual": {
                "session_token_data": {
                    "initiative": "web",
                    "certificate": "==",
                    "display_name": "applepay",
                    "certificate_keys": "",
                    "payment_processing_details_at": "Hyperswitch",
                    "payment_processing_certificate": "",
                    "payment_processing_certificate_key": "",
                    "initiative_context": "sdk-test-app.netlify.app",
                    "merchant_identifier": "",
                    "merchant_business_country": "US"
                },
                "payment_request_data": {
                    "label": "applepay",
                    "supported_networks": [
                        "visa",
                        "masterCard",
                        "amex",
                        "discover"
                    ],
                    "merchant_capabilities": [
                        "supports3DS"
                    ]
                }
            }
        },
        "google_pay": {
            "merchant_info": {
                "merchant_name": "Stripe"
            },
            "allowed_payment_methods": [
                {
                    "type": "CARD",
                    "parameters": {
                        "allowed_auth_methods": [
                            "PAN_ONLY",
                            "CRYPTOGRAM_3DS"
                        ],
                        "allowed_card_networks": [
                            "AMEX",
                            "DISCOVER",
                            "INTERAC",
                            "JCB",
                            "MASTERCARD",
                            "VISA"
                        ]
                    },
                    "tokenization_specification": {
                        "type": "PAYMENT_GATEWAY",
                        "parameters": {
                            "gateway": "stripe",
                            "stripe:version": "2018-10-31",
                            "stripe:publishableKey": "pk_test_51Msk2GAGHc77EJXX78h549SX2uaOnEkUYqBfjcoD05PIpAnDkYxMn8nQ4d19im85NQuX4Z6WDyHaUw2fFTPBWsIY00Wa7oNerO"
                        }
                    }
                }
            ]
        }
    }
}
```
<img width="1089" alt="image" src="https://github.com/user-attachments/assets/f312ee05-60be-41b7-bf2e-a0a83b15a524">

-> Also enable klarna via klarna
-> Now configure volume based routing for the connector
-> Create a payment and make a session call
```
curl --location 'http://localhost:8080/payments' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: dev_1l8FEkupRHGMJp9gZyj9HDUwgsSgAVgiKjtvdiDfocYs5Gjo2cn3UTpjNTBzYRHH' \
--data '{
  "amount": 10000,
  "currency": "USD",
  "capture_method": "automatic",
  "authentication_type": "no_three_ds",
  "confirm": false,
  "customer_id": "cu_1720854397"
}'
```
```
curl --location 'http://localhost:8080/payments/session_tokens' \
--header 'Content-Type: application/json' \
--header 'x-browser-name: Safari' \
--header 'x-client-platform: web' \
--header 'x-merchant-domain: sdk-test-app.netlify.app' \
--header 'api-key: pk_dev_2a79cad91c884bc7bb425a9822d09d0b' \
--data '{
    "payment_id": "pay_wp2gnnTNuK1oxMF2T6Yf",
    "wallets": [],
    "client_secret": "pay_wp2gnnTNuK1oxMF2T6Yf_secret_T10Mxvzq3hqBWfxOfIJI"
}'
```
<img width="1092" alt="image" src="https://github.com/user-attachments/assets/7c1af753-2cc9-46f0-bd01-bfc81ebeb81b">

-> Now disable klarna for stripe, create a payment and make a session call.
<img width="1152" alt="image" src="https://github.com/user-attachments/assets/19304947-400c-453e-bb6d-e2affe03927a">

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
