### 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.
name = r"<Name>\s*RedrivePolicy\s*<\/Name>"
        value = r"<Value>\s*{(.*)}\s*<\/Value>"
        for p1, p2 in ((name, value), (value, name)):
            content_str = re.sub(
                r"<Attribute>\s*%s\s*%s\s*<\/Attribute>" % (p1, p2),
                _fix_redrive_policy,
                content_str,
            )
        
        if action in (
            "CreateQueue",
            "GetQueueUrl",
            "ListQueues",
            "GetQueueAttributes",
            "ListDeadLetterSourceQueues",
        ):
            if config.USE_SSL and "<QueueUrl>http://" in content_str:
                
                content_str = re.sub(r"<QueueUrl>\s*http://", r"<QueueUrl>https://", content_str)
            
            external_port = SQS_PORT_EXTERNAL or get_external_port(headers, request_handler)
            content_str = re.sub(
                r"<QueueUrl>\s*([a-z]+)://[^<]*:([0-9]+)/([^<]*)\s*</QueueUrl>",
                r"<QueueUrl>\1://%s:%s/\3</QueueUrl>" % (config.HOSTNAME_EXTERNAL, external_port),
                content_str,
            )
            
            content_str = re.sub(
                r"<QueueUrl>\s*([a-z]+)://([^/]+)/queue/([^<]*)\s*</QueueUrl>",
                r"<QueueUrl>\1://\2/%s/\3</QueueUrl>" % constants.TEST_AWS_ACCOUNT_ID,