- [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

GATEWAY_RESPONSES[api_id] = GATEWAY_RESPONSES.get(api_id, [])
    data['responseType'] = response_type
    GATEWAY_RESPONSES[api_id].append(data)
    return data
def run_authorizer(api_id, headers, authorizer):
    
    pass
def authorize_invocation(api_id, headers):
    client = aws_stack.connect_to_service('apigateway')
    authorizers = client.get_authorizers(restApiId=api_id, limit=100).get('items', [])
    for authorizer in authorizers:
        run_authorizer(api_id, headers, authorizer)
def validate_api_key(api_key, stage):
    usage_plan_ids = []
    client = aws_stack.connect_to_service('apigateway')
    usage_plans = client.get_usage_plans()
    for item in usage_plans.get('items', []):
        api_stages = item.get('apiStages', [])
        for api_stage in api_stages:
            if api_stage.get('stage') == stage:
                usage_plan_ids.append(item.get('id'))
    for usage_plan_id in usage_plan_ids:
        usage_plan_keys = client.get_usage_plan_keys(usagePlanId=usage_plan_id)
        for key in usage_plan_keys.get('items', []):
            if key.get('value') == api_key:
                return True
    return False
def is_api_key_valid(is_api_key_required, headers, stage):