Allow empty notification responses

[camerski]

Issue #, if available: 133

Description of changes:
This change allows notification responses to be empty. Added unit tests to SlackDestinationTest, ChimeDestinationTest, and CustomWebhookMessageTest that cover null, empty, and non-empty response entities.

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 #272 into master will decrease coverage by 0.40%.
The diff coverage is 67.53%.

@@             Coverage Diff              @@
##             master     #272      +/-   ##
============================================
- Coverage     80.46%   80.06%   -0.41%     
+ Complexity      200      197       -3     
============================================
  Files           150      150              
  Lines          5099     5132      +33     
  Branches        670      661       -9     
============================================
+ Hits           4103     4109       +6     
- Misses          636      662      +26     
- Partials        360      361       +1     

Impacted Files	Coverage Δ	Complexity Δ
...opendistroforelasticsearch/alerting/model/Alert.kt	87.50% <ø> (ø)	0.00 <0.00> (ø)
...endistroforelasticsearch/alerting/model/Monitor.kt	88.70% <ø> (ø)	0.00 <0.00> (ø)
...icsearch/alerting/model/destination/Destination.kt	75.00% <ø> (ø)	0.00 <0.00> (ø)
...ting/destination/response/DestinationResponse.java	84.61% <0.00%> (ø)	2.00 <0.00> (ø)
...rting/transport/TransportIndexDestinationAction.kt	51.32% <25.00%> (-4.80%)	0.00 <0.00> (ø)
.../alerting/transport/TransportIndexMonitorAction.kt	59.87% <48.48%> (-10.20%)	0.00 <0.00> (ø)
...alerting/transport/TransportSearchMonitorAction.kt	51.42% <50.00%> (-11.08%)	0.00 <0.00> (ø)
...rch/alerting/transport/TransportGetAlertsAction.kt	72.60% <62.85%> (-8.65%)	0.00 <0.00> (ø)
...erting/transport/TransportGetDestinationsAction.kt	73.41% <68.08%> (-8.41%)	0.00 <0.00> (ø)
...orelasticsearch/alerting/model/MonitorRunResult.kt	85.12% <75.00%> (ø)	0.00 <0.00> (ø)
... and 18 more

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[skkosuri-amzn]

@camerski Thanks