Fix NPE in segment replicator

[andrross]

IndexStatsIT.testConcurrentIndexingAndStatsRequests sometimes fails with the following NPE:

IndexStatsIT > testConcurrentIndexingAndStatsRequests {p0={"cluster.indices.replication.strategy":"SEGMENT"}} FAILED
    java.lang.AssertionError:
    Expected: an empty collection
         but: <[[test][3] failed, reason [BroadcastShardOperationFailedException[operation indices:monitor/stats failed]; nested: NullPointerException[Cannot invoke "java.util.Map$Entry.getValue()" because "highestEntry" is null]; ]]>
        at __randomizedtesting.SeedInfo.seed([B56C9D0503BD16AE:2FDF6CC817B0190]:0)
        at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
        at org.junit.Assert.assertThat(Assert.java:964)
        at org.junit.Assert.assertThat(Assert.java:930)
        at org.opensearch.indices.stats.IndexStatsIT.testConcurrentIndexingAndStatsRequests(IndexStatsIT.java:1451)

The isEmpty() check on the concurrent map is not sufficient because entries can be removed after the check but before retrieving them.

This is one of the failures seen in #15836

Check List

• Functionality includes testing.

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[github-actions]

❌ Gradle check result for e466967: null

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 e466967: UNSTABLE

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]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.81%. Comparing base (9f13e37) to head (e466967).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...nsearch/indices/replication/SegmentReplicator.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18997      +/-   ##
============================================
- Coverage     72.88%   72.81%   -0.07%     
+ Complexity    69327    69311      -16     
============================================
  Files          5643     5645       +2     
  Lines        318720   318788      +68     
  Branches      46113    46125      +12     
============================================
- Hits         232294   232124     -170     
- Misses        67595    67878     +283     
+ Partials      18831    18786      -45     

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