3.x: Add nullability annotations to type arguments

[akarnokd]

Annotate type argument use with @NonNull.

Resolves #6766

[codecov]

Codecov Report

Merging #6840 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               3.x    #6840      +/-   ##
===========================================
- Coverage     98.3%   98.28%   -0.03%     
+ Complexity    6380     6379       -1     
===========================================
  Files          715      715              
  Lines        46529    46529              
  Branches      6388     6388              
===========================================
- Hits         45741    45729      -12     
- Misses         280      284       +4     
- Partials       508      516       +8

Impacted Files	Coverage Δ	Complexity Δ
...o/reactivex/rxjava3/processors/AsyncProcessor.java	98.95% <ø> (ø)	45 <0> (ø)	⬇️
...va/io/reactivex/rxjava3/plugins/RxJavaPlugins.java	100% <ø> (ø)	146 <0> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	179 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...reactivex/rxjava3/processors/PublishProcessor.java	100% <100%> (ø)	42 <0> (ø)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	99.01% <100%> (+2.46%)	50 <0> (ø)	⬇️
.../reactivex/rxjava3/processors/ReplayProcessor.java	97.79% <100%> (ø)	52 <0> (ø)	⬇️
...reactivex/rxjava3/processors/UnicastProcessor.java	100% <100%> (ø)	66 <0> (ø)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <100%> (ø)	576 <2> (ø)	⬇️
...activex/rxjava3/processors/MulticastProcessor.java	98.68% <100%> (+0.43%)	84 <0> (+1)	⬆️
... and 24 more

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