<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# Type of request: This is a ...

[x] bug report
[ ] feature request

# Detailed description

SNS is unable to forward the message to SQS (non fifo queue) when using the `elasticmq` backend and fails with the following log
```
2021-01-19T09:31:50:WARNING:localstack.services.sns.sns_listener: Unable to forward SNS message to SQS: An error occurred (InvalidParameterValue) when calling the SendMessage operation: Value for parameter MessageGroupId is invalid. The request include parameter that is not valid for this queue type Traceback (most recent call last):

File "/opt/code/localstack/localstack/services/sns/sns_listener.py", line 297, in message_to_subscribers

sqs_client.send_message(

File "/opt/code/localstack/.venv/lib/python3.8/site-packages/botocore/client.py", line 357, in _api_call

return self._make_api_call(operation_name, kwargs)

File "/opt/code/localstack/.venv/lib/python3.8/site-packages/botocore/client.py", line 676, in _make_api_call

raise error_class(parsed_response, operation_name)

botocore.exceptions.ClientError: An error occurred (InvalidParameterValue) when calling the SendMessage operation: Value for parameter MessageGroupId is invalid. The request include parameter that is not valid for this queue type
```
This is likely a regression due to #3435. Tested with the default backend on `0.12.5` and it works as expected. Also works with `elasticmq` backend on `0.12.4`

## Expected behavior

Same behavior with the `elasticmq` backend as with the default backend.

## Actual behavior

Fails on `elasticmq` backend, no SQS message received when SNS message is sent.

# Steps to reproduce

## Command used to start LocalStack

```yaml
  localstack:
    image: localstack/localstack:0.12.5
    ports:
      - 4566:4566
    environment:
      - AWS_DEFAULT_REGION=ap-northeast-1
      - SERVICES=sns,sqs
      - SQS_PROVIDER=elasticmq
```

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

LOG.info('Delivering SMS message to %s: %s', subscriber['Endpoint'], req_data['Message'][0])
        elif subscriber['Protocol'] == 'sqs':
            queue_url = None
            try:
                endpoint = subscriber['Endpoint']
                if 'sqs_queue_url' in subscriber:
                    queue_url = subscriber.get('sqs_queue_url')
                elif '://' in endpoint:
                    queue_url = endpoint
                else:
                    queue_name = endpoint.split(':')[5]
                    queue_url = aws_stack.get_sqs_queue_url(queue_name)
                    subscriber['sqs_queue_url'] = queue_url
                message_group_id = req_data.get('MessageGroupId')[0] if req_data.get('MessageGroupId') else ''
                sqs_client = aws_stack.connect_to_service('sqs')
                sqs_client.send_message(
                    QueueUrl=queue_url,
                    MessageBody=create_sns_message_body(subscriber, req_data, message_id),
                    MessageAttributes=create_sqs_message_attributes(subscriber, message_attributes),
                    MessageGroupId=message_group_id
                )
            except Exception as exc:
                LOG.warning('Unable to forward SNS message to SQS: %s %s' % (exc, traceback.format_exc()))
                sns_error_to_dead_letter_queue(subscriber['SubscriptionArn'], req_data, str(exc))
                if 'NonExistentQueue' in str(exc):
                    LOG.info('Removing non-existent queue "%s" subscribed to topic "%s"' % (queue_url, topic_arn))
                    subscriptions.remove(subscriber)
        elif subscriber['Protocol'] == 'lambda':
            try:
                external_url = external_service_url('sns')