For query protocols ignore the body if the output schema has the unit type trait (#1173)

Some services such as Neptune's AddTagsToResourceResponse operation
that targets `smithy.api#Unit` for its output still returns the
`ResponseMetadata` in the payload. This change ignores the response
when the operation output targes Unit. See example of output below.

```
  <AddTagsToResourceResponse xmlns="http://rds.amazonaws.com/doc/2014-10-31/">
    <ResponseMetadata>
      <RequestId>abc-123</RequestId>
    </ResponseMetadata>
  </AddTagsToResourceResponse
```

Author: sugmanue

aws/client/aws-client-awsquery/src/main/java/software/amazon/smithy/java/aws/client/awsquery/AwsQueryClientProtocol.java

@@ -102,7 +102,7 @@ public <I extends SerializableStruct, O extends SerializableStruct> O deserializ
         var builder = operation.outputBuilder();
         var content = response.body();
 
-        if (content.contentLength() == 0) {
+        if (content.contentLength() == 0 || operation.outputSchema().hasTrait(TraitKey.UNIT_TYPE_TRAIT)) {
             return builder.build();
         }