### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Starting the localstack docker using the elasticmq SQS backend results in not being able to send any messages.  Appears to work fine in 0.12.17

### Expected Behavior

Messages should send fine.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

Run docker with:
```
docker run --rm -it -p 4566:4566 --env SERVICES=sqs --env SQS_PROVIDER=elasticmq --env DEBUG=1 localstack/localstack:0.12.18
```
Execute a simple SQS send:
```
AWS_ACCESS_KEY_ID=x AWS_SECRET_ACCESS_KEY=y aws --endpoint-url=http://localhost:4566 sqs create-queue --queue-name foo && aws --endpoint-url=http://localhost:4566 sqs send-message --queue-url "http://localhost:4658/000000000000/foo" --message-body foo
```
results in failure:
```
Unable to parse response (not well-formed (invalid token): line 1, column 0), invalid XML received. Further retries may succeed:
b'{}'
```

### Environment

```markdown
- OS: macOS 11.6
- LocalStack: 0.12.18
```


### Anything else?

I'm pretty sure that commit e38778888534cb6d98f27edaf5af76c1e1d29e06 broke the functionality.  I _think_ the `_process_sent_message` function is assuming something about the response that just isn't true, but I haven't gotten any further in my analysis.

Also, I need to use elasticmq because the moto implementation is just not fast enough.  We've been using an older release (0.11.3), using elasticmq, and I can send and receive 10,000 2K bodies in about 25 seconds, but using 0.12.18 and moto I can only send about 1200 and receive about 450 in those 25 seconds, and the slowdown just isn't acceptable for our integration tests.
queue_url = req_data.get("QueueUrl")
    if queue_url:
        return queue_url
    url = config.TEST_SQS_URL
    if headers.get("Host"):
        url = "%s://%s" % (get_service_protocol(), headers["Host"])
    queue_url = "%s%s" % (url, path.partition("?")[0])
    return queue_url
def _list_dead_letter_source_queues(queues, queue_url):
    dead_letter_source_queues = []
    for k, v in queues.items():
        for i, j in v.items():
            if i == "RedrivePolicy":
                f = json.loads(v[i])
                queue_url_split = queue_url.split("/")
                if queue_url_split[-1] in f["deadLetterTargetArn"]:
                    dead_letter_source_queues.append(k)
    return format_list_dl_source_queues_response(dead_letter_source_queues)
def _process_sent_message(path: str, req_data: Dict[str, str], headers: Dict, response: Response):