[Backport 2.x] Fix setting default title only when no subject has been set

[opensearch-trigger-bot]

Backport 830dff1 from #750

[codecov-commenter]

Codecov Report

Merging #785 (7dfd92e) into 2.x (4ed3c0c) will decrease coverage by 0.10%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##                2.x     #785      +/-   ##
============================================
- Coverage     75.61%   75.52%   -0.10%     
  Complexity      116      116              
============================================
  Files           125      125              
  Lines          6857     6851       -6     
  Branches       1031     1028       -3     
============================================
- Hits           5185     5174      -11     
- Misses         1146     1152       +6     
+ Partials        526      525       -1     

Impacted Files	Coverage Δ
...in/kotlin/org/opensearch/alerting/MonitorRunner.kt	63.29% <0.00%> (ø)
.../util/destinationmigration/NotificationApiUtils.kt	62.50% <0.00%> (+5.35%)	⬆️
.../alerting/transport/TransportIndexMonitorAction.kt	54.27% <0.00%> (-2.97%)	⬇️
.../kotlin/org/opensearch/alerting/util/IndexUtils.kt	70.83% <0.00%> (-2.09%)	⬇️
...rg/opensearch/alerting/BucketLevelMonitorRunner.kt	79.13% <0.00%> (-0.44%)	⬇️

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