Apply minor cleanups suggested by IntelliJ in generics code

[msridhar]

Remove unnecessary casts, switch to isEmpty() in a couple of places, fix a typo. No behavior changes, just cleanup

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9092438) 87.02% compared to head (c549ea9) 87.02%.

Files	Patch %	Lines
...ava/com/uber/nullaway/generics/GenericsChecks.java	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #860      +/-   ##
============================================
- Coverage     87.02%   87.02%   -0.01%     
  Complexity     1921     1921              
============================================
  Files            77       77              
  Lines          6219     6218       -1     
  Branches       1209     1209              
============================================
- Hits           5412     5411       -1     
  Misses          403      403              
  Partials        404      404              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[yuxincs]

Nice cleanup!