[Backport 2.x] Fix GetSnapshots to not return non-existent snapshots with ignore_unavailable=true

[opensearch-trigger-bot]

Backport 8b34e5f from #6839.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollWithOngoingSegmentReplication
      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=pit/10_basic/Delete all}

• URL: https://build.ci.opensearch.org/job/gradle-check/13652/
• CommitID: e32371e
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov-commenter]

Codecov Report

Merging #7029 (e32371e) into 2.x (d95effb) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##                2.x    #7029      +/-   ##
============================================
- Coverage     70.41%   70.33%   -0.08%     
+ Complexity    59535    59491      -44     
============================================
  Files          4822     4822              
  Lines        285910   285910              
  Branches      41551    41552       +1     
============================================
- Hits         201319   201093     -226     
- Misses        67766    68022     +256     
+ Partials      16825    16795      -30     

Impacted Files	Coverage Δ
...ter/snapshots/get/TransportGetSnapshotsAction.java	41.58% <0.00%> (ø)

... and 495 files with indirect coverage changes

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