### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

There is an issue with `get-object-attributes` command which results in below error always: 
`aws --endpoint-url=http://localhost:4566 s3api get-object-attributes --bucket s3mock --key 20.txt --object-attributes "ObjectSize"`
````

> Response:  Unable to parse response (syntax error: line 1, column 0), invalid XML received. Further retries may succeed:
b'98\n5\n'
````

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### Using docker-compse.yaml to start localstack

docker-compse.yaml:

```
localstack:
    image: localstack/localstack:latest
    ports:
      - '4563-4599:4563-4599'
      - '8055:8080'
    environment:
      - SERVICES=s3
      - DEBUG=1
      - LS_LOG=error
      - DATA_DIR=/tmp/localstack/data
      - AWS_DEFAULT_REGION=eu-west-1
      # - EDGE_PORT=4566
    volumes:
      - "./.localstack:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
```

#### cli commands to reproduce the issue

1. Create a Bucket: `aws --endpoint-url=http://localhost:4566 s3 mb s3://s3mock`
2. Create an Object: `aws --endpoint-url=http://localhost:4566 s3 cp data/20.txt s3://s3mock`

File 20.txt content are 2-digit numbers separated by newline.

3. get-object-attributes for object 20.txt :
```

 aws --endpoint-url=http://localhost:4566 s3api get-object-attributes --bucket s3mock --key 20.txt --object-attributes "ObjectSize"

```
Results in error:

> Unable to parse response (syntax error: line 1, column 0), invalid XML received. Further retries may succeed:
b'98\n5\n'



### Environment

```markdown
- OS:Mac 12.x
- LocalStack:latest
- Node 14
```


### Anything else?

_No response_
value: Optional[dict],
        
    ) -> Optional[dict]:
        if shape.is_document_type:
            final_parsed = value
        else:
            if value is None:
                # If the comes across the wire as "null" (None in python),
                # we should be ning this unchanged, ins
                # empty dict.
                n None
            final_parsed = {}
            for member_name, member_shape in shape.members.items():
                json_name = member_shape.ization.get("name", member_name)
                raw_value = value.get(json_name)
                parsed = self._parse_shape(request, member_shape, raw_value, uri_params)
                if parsed is not None or member_name in shape.required_members:
                    # If the member is required, but not existing, we set it to None anyways
                    final_parsed[member_name] = parsed
        n final_parsed
    def _parse_map(
        self,
        request: HttpRequest,
        shape: MapShape,
        value: Optional[dict],
        
    ) -> Optional[dict]:
        if value is None:
            n None
        parsed = {}