Fix NullPointerException when a reviewer is deleted (#771)

[sahansera]

Description

This PR was created to add a fix to cover the following scenarios.

• User A requests a PR review from User B. User B gets deleted. GitHub removes the requested reviewer from the PR.
• User A requests a PR review from a User B. User A gets deleted. GitHub assigns a ghost account to User A.
• Also includes an additional test checkPullRequestReviewer to improve coverage.

Fixes #771

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[codecov]

Codecov Report

Merging #1318 (614ffcc) into main (7df326e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1318      +/-   ##
============================================
+ Coverage     78.02%   78.04%   +0.02%     
- Complexity     2027     2029       +2     
============================================
  Files           199      199              
  Lines          6243     6245       +2     
  Branches        356      356              
============================================
+ Hits           4871     4874       +3     
+ Misses         1162     1161       -1     
  Partials        210      210              

Impacted Files	Coverage Δ
...n/java/org/kohsuke/github/GHPullRequestReview.java	70.00% <100.00%> (+4.21%)	⬆️

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[bitwiseman]

@sahansera
I edited your description to say "Fixes #771". This will cause that issue to be automatically closed when this PR is merged.