Add Nullable annotations for blocking methods in Completable

[Oknesif]

Just add @nullable annotations to blockingGet() methods in Completable class.

[codecov]

Codecov Report

Merging #6244 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6244      +/-   ##
============================================
+ Coverage     98.25%   98.25%   +<.01%     
- Complexity     6199     6204       +5     
============================================
  Files           667      667              
  Lines         44887    44887              
  Branches       6216     6216              
============================================
+ Hits          44104    44105       +1     
- Misses          243      248       +5     
+ Partials        540      534       -6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-4.58%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-3.87%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	98.2% <0%> (-0.9%)	59% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
...ex/internal/operators/flowable/FlowableReplay.java	94.26% <0%> (-0.85%)	20% <0%> (ø)
...x/internal/operators/flowable/FlowableGroupBy.java	95.51% <0%> (-0.57%)	3% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	97.1% <0%> (-0.49%)	8% <0%> (ø)
...va/io/reactivex/processors/MulticastProcessor.java	98.23% <0%> (-0.45%)	83% <0%> (-1%)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.69% <0%> (-0.21%)	50% <0%> (ø)
... and 15 more

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