JSpecify: changes for issue #861

[akulk022]

Removing unnecessary checks performed for ClassType in GenericChecks.java in reference to issue #861. Added some null checks since some of the checks for ClassType were also indirectly checking for nullability and hence removing them entirely cause the test rawTypes to crash.

Fixes #861

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (eb2e19c) 87.05% compared to head (f3b8129) 87.06%.

Files	Patch %	Lines
...ava/com/uber/nullaway/generics/GenericsChecks.java	30.00%	0 Missing and 7 partials ⚠️
...r/nullaway/generics/CompareNullabilityVisitor.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #863   +/-   ##
=========================================
  Coverage     87.05%   87.06%           
- Complexity     1923     1924    +1     
=========================================
  Files            77       77           
  Lines          6221     6225    +4     
  Branches       1208     1209    +1     
=========================================
+ Hits           5416     5420    +4     
  Misses          403      403           
  Partials        402      402           

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