- [x] bug report
- [ ] feature request


# Detailed description

Creating an api gateway mock integration response and calling it results in an internal server error:

```
2021-04-20T22:21:22:WARNING:localstack.utils.server.http2_server: Error in proxy handler for request OPTIONS http://localhost:4566/restapis/qf25a1oyzk/stage/_user_request_/state/: 'NoneType' object has no attribute 'startswith' Traceback (most recent call last):
  File "/opt/code/localstack/localstack/utils/server/http2_server.py", line 129, in index
    raise result
  File "/opt/code/localstack/localstack/utils/bootstrap.py", line 681, in run
    result = self.func(self.params)
  File "/opt/code/localstack/localstack/utils/async_utils.py", line 28, in _run
    return fn(*args, **kwargs)
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 352, in handler
    response = modify_and_forward(method=method, path=path_with_params, data_bytes=data, headers=headers,
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 171, in modify_and_forward
    listener_result = listener.forward_request(method=method,
  File "/opt/code/localstack/localstack/services/edge.py", line 117, in forward_request
    return do_forward_request(api, method, path, data, headers, port=port)
  File "/opt/code/localstack/localstack/services/edge.py", line 136, in do_forward_request
    result = do_forward_request_inmem(api, method, path, data, headers, port=port)
  File "/opt/code/localstack/localstack/services/edge.py", line 156, in do_forward_request_inmem
    response = modify_and_forward(method=method, path=path, data_bytes=data, headers=headers,
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 171, in modify_and_forward
    listener_result = listener.forward_request(method=method,
  File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 48, in forward_request
    return invoke_rest_api_from_request(method, path, data, headers)
  File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 256, in invoke_rest_api_from_request
    return invoke_rest_api(api_id, stage, method, relative_path_w_query_params,
  File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 294, in invoke_rest_api
    return invoke_rest_api_integration(api_id, stage, integration, method, path, invocation_path, data,
  File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 307, in invoke_rest_api_integration
    if (uri.startswith('arn:aws:apigateway:') and ':lambda:path' in uri) or uri.startswith('arn:aws:lambda'):
AttributeError: 'NoneType' object has no attribute 'startswith'
```

Looks like this regression was introduced in https://github.com/localstack/localstack/pull/3539. That fits the timeline when my project stopped building.

## Expected behavior
The mock response is returned


## Actual behavior
500 Internal Server Error


# Steps to reproduce

Create a cors endpoint with type mock and no uri (I used https://github.com/bridgecrewio/terraform-aws-apigateway-cors/)

## Command used to start LocalStack

docker compose using newest localstack

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

terraform apply

import re
import json
import time
import logging
import requests
import datetime
from flask import Response as FlaskResponse
from requests.models import Response
from six.moves.urllib_parse import urljoin
from moto.apigateway.models import apigateway_backends
from localstack.utils import common
from localstack.config import TEST_KINESIS_URL, TEST_SQS_URL
from localstack.constants import APPLICATION_JSON, PATH_USER_REQUEST, TEST_AWS_ACCOUNT_ID
from localstack.utils.aws import aws_stack, aws_responses
from localstack.utils.common import to_str, to_bytes
from localstack.utils.analytics import event_publisher
from localstack.services.kinesis import kinesis_listener
from localstack.services.awslambda import lambda_api
from localstack.services.apigateway import helpers
from localstack.services.generic_proxy import ProxyListener
from localstack.utils.aws.aws_responses import flask_to_requests_response, requests_response, LambdaResponse
from localstack.services.apigateway.helpers import (get_resource_for_path, handle_authorizers, handle_validators,
    handle_accounts, extract_query_string_params, extract_path_params, make_error_response, get_cors_response,
    hande_base_path_mappings)
LOGGER = logging.getLogger(__name__)