- [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

if uri.startswith('arn:aws:apigateway:') and ':sqs:path' in uri:
                template = integration['requestTemplates'][APPLICATION_JSON]
                account_id, queue = uri.split('/')[-2:]
                region_name = uri.split(':')[3]
                new_request = '%s&QueueName=%s' % (aws_stack.render_velocity_template(template, data), queue)
                headers = aws_stack.mock_aws_request_headers(service='sqs', region_name=region_name)
                url = urljoin(TEST_SQS_URL, '%s/%s' % (TEST_AWS_ACCOUNT_ID, queue))
                result = common.make_http_request(url, method='POST', headers=headers, data=new_request)
                return result
        msg = 'API Gateway AWS integration action URI "%s", method "%s" not yet implemented' % (uri, method)
        LOGGER.warning(msg)
        return make_error_response(msg, 404)
    elif integration_type == 'AWS_PROXY':
        if uri.startswith('arn:aws:apigateway:') and ':dynamodb:action' in uri:
            
            table_name = uri.split(':dynamodb:action')[1].split('&Table=')[1]
            action = uri.split(':dynamodb:action')[1].split('&Table=')[0]
            if 'PutItem' in action and method == 'PUT':
                response_template = response_templates.get('application/json', None)
                if response_template is None:
                    msg = 'Invalid response template defined in integration response.'
                    return make_error_response(msg, 404)
                response_template = json.loads(response_template)
                if response_template['TableName'] != table_name:
                    msg = 'Invalid table name specified in integration response template.'
                    return make_error_response(msg, 404)
                dynamo_client = aws_stack.connect_to_resource('dynamodb')
                table = dynamo_client.Table(table_name)
                event_data = {}
                data_dict = json.loads(data)