Fix 4990, allow null values in S3 object metadata

[dropofwill]

Change tests to check keys in any order instead of relying on keySets implicit ordering.

Motivation and Context

Fixes #4990

Modifications

Collect with HashMap instead of built in Java collector which cannot handle nulls.

Testing

Added a test with null keys, fails without my change, passes with it.

Checklist

• [x] I have read the CONTRIBUTING document
• [x] Local run of mvn install succeeds
• [x] 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
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed
• [x] 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

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