Fix NPE issue in multipart S3 client

[zoewangg]

Motivation and Context

Fix NPE issue thrown when using multipart S3 client to upload an object containing empty content without supplying a content length. Fix #6464

When uploading an empty object with unknown content length using the S3 multipart client, a NullPointerException was thrown because the code didn't handle the case
where no async request body chunks are received before the stream completes.

Modifications

• Modified UploadWithUnknownContentLengthHelper to handle empty streams by checking if any async request body has been received before onComplete() is called
• Added null check in onNext() to fail gracefully if a null async request body is passed
• Renamed isFirstAsyncRequestBody to firstAsyncRequestBodyReceived for better clarity
• When no request body is received, use AsyncRequestBody.empty() for single object upload

Testing

• Added new tests

Types of changes

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

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.3% Coverage on New Code
0.0% Duplication on New Code

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