Set explicit payload members to null if the input is empty

[sugmanue]

Motivation and Context

Fixes a regression for the REST JSON protocol. Before a explicit payload member of the response was unmarshalled as null when the response body was empty, and after enabling the fast JSON unmarshaller this behavior changed to unmarshall it as an POJO without any members set.

This change reverts to the previous behavior and now the member is unmarshalled as null.

Modifications

Testing

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[sonarqubecloud]

Quality Gate failed

Failed conditions
73.1% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[sugmanue]

Protocol tests for Smithy for this case is pending review, see #2641.