Do not throw exception when flat_object field is explicitly null

[msfroh]

Description

It is valid for a flat_object field to have an explicit value of null. (It's functionally the same as not specifying the field at all.) Prior to this fix, though, we would erroneously advance the context parser to the next token, violating the contract with DocumentParser (which says that a call to parseCreateField with a null value should complete with the parser still pointing at the null value -- it is DocumentParser's responsibility to advance).

Related Issues

N/A

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for b416205: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 0a952c3: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.87%. Comparing base (9014894) to head (0539f08).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15375      +/-   ##
============================================
- Coverage     71.96%   71.87%   -0.09%     
+ Complexity    63319    63203     -116     
============================================
  Files          5224     5224              
  Lines        296138   296137       -1     
  Branches      42777    42777              
============================================
- Hits         213110   212863     -247     
- Misses        65546    65733     +187     
- Partials      17482    17541      +59     

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[reta]

Could you please add changelog entry? seems like a bugfix

[msfroh]

Could you please add changelog entry? seems like a bugfix

Sure!

[github-actions]

✅ Gradle check result for 3d6e908: SUCCESS

[github-actions]

❕ Gradle check result for 0539f08: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.http.SearchRestCancellationIT.testAutomaticCancellationDuringFetchPhase

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-15375-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 46a269ef21e88e3cb1398474e4bf82af4c3b3b7f
# Push it to GitHub
git push --set-upstream origin backport/backport-15375-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-15375-to-2.x.