Fix 1/ IllegalStateException warn messages, 2/ location header in destination response, 3/ handle null in GetMonitorRequest

[skkosuri-amzn]

Issue #, if available:
n/a
Description of changes:

1. IllegalStateException warn messages
2. location header in destination response,
3. handle null in GetMonitorRequest

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #252 into master will increase coverage by 0.28%.
The diff coverage is 48.48%.

@@             Coverage Diff              @@
##             master     #252      +/-   ##
============================================
+ Coverage     78.42%   78.70%   +0.28%     
- Complexity      159      164       +5     
============================================
  Files            91       91              
  Lines          3249     3273      +24     
  Branches        458      464       +6     
============================================
+ Hits           2548     2576      +28     
+ Misses          462      454       -8     
- Partials        239      243       +4     

Impacted Files	Coverage Δ	Complexity Δ
...rting/transport/TransportIndexDestinationAction.kt	51.72% <27.27%> (-3.98%)	0.00 <0.00> (ø)
.../alerting/transport/TransportIndexMonitorAction.kt	62.09% <33.33%> (-4.00%)	0.00 <0.00> (ø)
...elasticsearch/alerting/action/GetMonitorRequest.kt	96.00% <85.71%> (+1.00%)	0.00 <0.00> (ø)
...alerting/resthandler/RestIndexDestinationAction.kt	90.62% <100.00%> (+6.25%)	0.00 <0.00> (ø)
...lerting/transport/TransportExecuteMonitorAction.kt	75.00% <100.00%> (+10.48%)	0.00 <0.00> (ø)
...ch/alerting/transport/TransportGetMonitorAction.kt	85.71% <100.00%> (+0.71%)	0.00 <0.00> (ø)
...e/action/node/ScheduledJobsStatsTransportAction.kt	75.00% <0.00%> (ø)	8.00% <0.00%> (ø%)
...asticsearch/alerting/core/schedule/JobScheduler.kt	75.71% <0.00%> (+4.28%)	15.00% <0.00%> (ø%)
... and 3 more

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