2.x: Add a few more @Nullable & @NonNull annotations to public interfaces

[vanniktech]

• adds the @nullable & @nonnull annotations to
  • Emitters
  • Observer
  • OnSubscribe
  • Operator
  • Source
  • Transformer

for Observable, Maybe, Single, Completable & Flowable.

[codecov]

Codecov Report

Merging #5196 into 2.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5196      +/-   ##
============================================
+ Coverage     95.96%   96.04%   +0.08%     
+ Complexity     5679     5677       -2     
============================================
  Files           621      621              
  Lines         40611    40611              
  Branches       5632     5632              
============================================
+ Hits          38971    39004      +33     
+ Misses          652      631      -21     
+ Partials        988      976      -12

Impacted Files	Coverage Δ	Complexity Δ
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	95.58% <0%> (-2.95%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-2.84%)	31% <0%> (-2%)
...java/io/reactivex/subjects/CompletableSubject.java	94.8% <0%> (-2.6%)	35% <0%> (-1%)
.../main/java/io/reactivex/subjects/AsyncSubject.java	97.41% <0%> (-1.73%)	55% <0%> (-1%)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
...n/java/io/reactivex/processors/AsyncProcessor.java	97.45% <0%> (-1.7%)	55% <0%> (-1%)
...ors/observable/ObservableSampleWithObservable.java	97.59% <0%> (-1.21%)	3% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	92.04% <0%> (-1.14%)	24% <0%> (-1%)
...al/operators/flowable/FlowableSamplePublisher.java	95.74% <0%> (-1.07%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.02% <0%> (-1%)	2% <0%> (ø)
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7265d33...c165522. Read the comment docs.