Fix NPE getting metaFields from mapperService on a close index request

[cwperks]

Description

Fixes a NullPointerException that occurs on a Closed index request. This fixes an issue seen after the merge of #4370 which made a change to get the list of metaFields from the mapperService in core. On close index requests, indexService.mapperService() can return null so the call to indexService.mapperService().getMetadataFields() throws an NPE.

mapperService would be null because needsMapperService would return false on closed index.

This fix checks for Close index requests and handles the scenario accordingly. Closed indices are not searchable so there's no need to have a list of metaFields for a closed index.

The close index API operation closes an index. Once an index is closed, you cannot add data to it or search for any data within the index.

Ref: https://opensearch.org/docs/latest/api-reference/index-apis/close-index/

• Category

Bug fix

Issues Resolved

• Investigate ISM failures caused during 2.15 RC generation for a bug fix to remove static meta-fields #4475

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.21%. Comparing base (6dedfb4) to head (0a6cde9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4497      +/-   ##
==========================================
- Coverage   65.21%   65.21%   -0.01%     
==========================================
  Files         313      313              
  Lines       22047    22053       +6     
  Branches     3559     3561       +2     
==========================================
+ Hits        14378    14381       +3     
- Misses       5895     5897       +2     
- Partials     1774     1775       +1     

Files	Coverage Δ
...figuration/SecurityFlsDlsIndexSearcherWrapper.java	91.66% <70.00%> (-5.96%)	⬇️

[shikharj05]

good catch, thank you fixing this!

[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/security/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/security/backport-2.x
# Create a new branch
git switch --create backport/backport-4497-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 34da6e86fbcdaf2b87a3dadf33e1c53e425b280e
# Push it to GitHub
git push --set-upstream origin backport/backport-4497-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/security/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-4497-to-2.x.

[cwperks]

Creating a manual backport