Bug: [BUG] Fix Deserialization error in Pm auth core



### Feature Description

Currently, if we try to link accounts twice in pm auth flow, the `/exchange` call fails due to a deserialization error.
This happens because of extra entries created in DB during mandate save pm flow.

### Possible Implementation

Need to check on pm data first before deserialization.

### Have you spent some time checking if this feature request has been raised before?

- [X] I checked and didn't find a similar issue

### Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

Yes, I am willing to submit a PR!
diff --git a/crates/router/src/core/pm_auth.rs b/crates/router/src/core/pm_auth.rs
index 64c1918c5e5..47f843030ee 100644
--- a/crates/router/src/core/pm_auth.rs
+++ b/crates/router/src/core/pm_auth.rs
@@ -346,7 +346,9 @@ async fn store_bank_details_in_payment_methods(
     > = HashMap::new();
 
     for pm in payment_methods {
-        if pm.payment_method == Some(enums::PaymentMethod::BankDebit) {
+        if pm.payment_method == Some(enums::PaymentMethod::BankDebit)
+            && pm.payment_method_data.is_some()
+        {
             let bank_details_pm_data = crypto_operation::<serde_json::Value, masking::WithType>(
                 &(&state).into(),
                 type_name!(storage::PaymentMethod),

## 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 -->
Earlier, the mandate request was inserting another entry in payment_methods table which interfered with pm_auth when running it again. The `/auth/exchange` call was failing due to this. 
Have fixed the logic in pm_auth to be more specific.

### 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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2. `crates/router/src/configs`
3. `loadtest/config`
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## Motivation and Context
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## How did you test it?
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Can be tested via SDK
1. Test Pm auth flow (with `  "setup_future_usage": "off_session"` passed in `payment_create` and `customer_acceptance` passed in `payment_confirm`)
2. Retest the Pm auth flow

## 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`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
