### 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.
import json
import re
from typing import Dict
import xmltodict
from moto.sqs.models import TRANSPORT_TYPE_ENCODINGS, Message
from moto.sqs.utils import parse_message_attributes
from requests.models import Request, Response
from six.moves.urllib.parse import urlencode
from localstack import config, constants
from localstack.config import SQS_PORT_EXTERNAL
from localstack.services.awslambda.lambda_api import EventSourceListener
from localstack.services.install import SQS_BACKEND_IMPL
from localstack.services.sns import sns_listener
from localstack.utils.analytics import event_publisher
from localstack.utils.aws import aws_stack
from localstack.utils.aws.aws_responses import (
    calculate_crc32,
    make_requests_error,