Nonnull on events

[KostyaSha]

fixes #210

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This change is 

[codecov-io]

Codecov Report

Merging #216 into master will increase coverage by 0.2%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master     #216     +/-   ##
===========================================
+ Coverage     54.65%   54.85%   +0.2%     
- Complexity      605      610      +5     
===========================================
  Files           113      113             
  Lines          2966     2966             
  Branches        243      243             
===========================================
+ Hits           1621     1627      +6     
+ Misses         1256     1247      -9     
- Partials         89       92      +3

Impacted Files	Coverage Δ	Complexity Δ
...ci/plugins/github/pullrequest/GitHubPRTrigger.java	66.37% <0%> (-0.87%)	18 <1> (ø)
...github/integration/branch/GitHubBranchTrigger.java	61.81% <0%> (-0.91%)	17 <1> (ø)
...lugins/github/pullrequest/GitHubPRPullRequest.java	78.87% <0%> (+1.4%)	28% <0%> (+1%)	⬆️
...webhook/GitHubBranchRepositoryNameContributor.java	60% <0%> (+60%)	2% <0%> (+2%)	⬆️
...est/webhook/GitHubPRRepositoryNameContributor.java	80% <0%> (+80%)	2% <0%> (+2%)	⬆️

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