### 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.
parse_urlencoded_data,
    requests_response,
)
from localstack.utils.common import (
    clone,
    ensure_list,
    get_service_protocol,
    parse_request_data,
    path_from_url,
    to_str,
)
from localstack.utils.persistence import PersistingProxyListener
API_VERSION = "2012-11-05"
XMLNS_SQS = "http://queue.amazonaws.com/doc/%s/" % API_VERSION
MSG_CONTENT_REGEX = "^[\u0009\u000A\u000D\u0020-\uD7FF\uE000-\uFFFD\U00010000-\U0010FFFF]*$"
UNSUPPORTED_ATTRIBUTE_NAMES = [
    
    
    "FifoQueue",
    "ContentBasedDeduplication",
    "DelaySeconds",
    "MaximumMessageSize",
    "MessageRetentionPeriod",
    "Policy",
    "RedrivePolicy",
    "KmsMasterKeyId",
    "KmsDataKeyReusePeriodSeconds",
]
QUEUE_ATTRIBUTES = {}
def _format_attributes(req_data):