[Backport 2.x] Fix null values indexed as "null" strings in flat_object field

[opensearch-trigger-bot]

Backport 9194b7a from #14069.

[github-actions]

❌ Gradle check result for 1f17544: 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 1f17544: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.

Project coverage is 71.55%. Comparing base (3c9e08b) to head (1f17544).
Report is 6 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...ch/common/xcontent/JsonToStringXContentParser.java	60.60%	6 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #15515      +/-   ##
============================================
- Coverage     71.60%   71.55%   -0.05%     
+ Complexity    63549    63493      -56     
============================================
  Files          5222     5222              
  Lines        297639   297660      +21     
  Branches      43325    43331       +6     
============================================
- Hits         213112   212992     -120     
- Misses        66754    66806      +52     
- Partials      17773    17862      +89     

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