Nullability annotations

[Jawnnypoo]

This adds more nullability annotations to ParseObject and also cleans up lint warnings in that class, and removes some dead links from the code

[codecov]

Codecov Report

Merging #857 into master will increase coverage by 0.06%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##             master     #857      +/-   ##
============================================
+ Coverage     64.46%   64.53%   +0.06%     
- Complexity     1905     1907       +2     
============================================
  Files           123      123              
  Lines          9788     9779       -9     
  Branches       1374     1373       -1     
============================================
+ Hits           6310     6311       +1     
+ Misses         2970     2960      -10     
  Partials        508      508

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/parse/CachedCurrentUserController.java	93.96% <ø> (ø)	15 <0> (ø)	⬇️
...com/parse/CachedCurrentInstallationController.java	96.61% <ø> (ø)	11 <0> (ø)	⬇️
parse/src/main/java/com/parse/ParseRole.java	100% <ø> (ø)	17 <0> (ø)	⬇️
parse/src/main/java/com/parse/ParseUser.java	79.59% <ø> (ø)	117 <0> (ø)	⬇️
parse/src/main/java/com/parse/ParseObject.java	59.71% <72.72%> (+0.16%)	234 <6> (ø)	⬇️
parse/src/main/java/com/parse/PLog.java	76.66% <0%> (+13.33%)	13% <0%> (+2%)	⬆️

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[coveralls]

Coverage increased (+0.07%) to 69.731% when pulling 18f17ef on Jawnnypoo:nullability into 8f3a2d8 on parse-community:master.