Fix parsing User info when User is null

[lezzago]

Issue #, if available:

Description of changes:
Set User to null when parsing null User data from Monitor, Destination, or Alert object.

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

[skkosuri-amzn]

thanks @lezzago

[codecov]

Codecov Report

Merging #279 into master will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #279      +/-   ##
============================================
+ Coverage     79.28%   79.57%   +0.29%     
- Complexity      193      194       +1     
============================================
  Files           150      150              
  Lines          5132     5132              
  Branches        661      664       +3     
============================================
+ Hits           4069     4084      +15     
+ Misses          708      696      -12     
+ Partials        355      352       -3     

Impacted Files	Coverage Δ	Complexity Δ
...opendistroforelasticsearch/alerting/model/Alert.kt	87.50% <100.00%> (ø)	0.00 <0.00> (ø)
...endistroforelasticsearch/alerting/model/Monitor.kt	88.70% <100.00%> (ø)	0.00 <0.00> (ø)
...icsearch/alerting/model/destination/Destination.kt	75.00% <100.00%> (ø)	0.00 <0.00> (ø)
...distroforelasticsearch/alerting/util/IndexUtils.kt	71.42% <0.00%> (-2.39%)	0.00% <0.00%> (ø%)
...ticsearch/alerting/elasticapi/ElasticExtensions.kt	51.92% <0.00%> (+3.84%)	0.00% <0.00%> (ø%)
...ing/destination/client/DestinationEmailClient.java	77.50% <0.00%> (+5.00%)	8.00% <0.00%> (+1.00%)
...roforelasticsearch/alerting/alerts/AlertIndices.kt	75.59% <0.00%> (+9.44%)	0.00% <0.00%> (ø%)

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