[Backport 2.x] Fix exists queries on nested flat_object fields throws exception

[kkewwei]

Description

Backport b7d7cf9 from #16803

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

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 bffc9a3: 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 c48e25f: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.03%. Comparing base (a79c6e8) to head (c48e25f).
Report is 5 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #17225      +/-   ##
============================================
+ Coverage     71.98%   72.03%   +0.05%     
- Complexity    65956    65967      +11     
============================================
  Files          5341     5341              
  Lines        307157   307162       +5     
  Branches      44823    44825       +2     
============================================
+ Hits         221110   221269     +159     
+ Misses        67618    67388     -230     
- Partials      18429    18505      +76     

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