Added nullability annotation for RxJavaPlugins.onCompletableAssembly

[afeozzz]

Just annotation that didn't exist

[vanniktech]

Do we also need the @SuppressWarnings("rawtypes") here?

[akarnokd]

Completable does not have a type argument.

[codecov]

Codecov Report

Merging #5838 into 2.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5838      +/-   ##
============================================
- Coverage     96.39%   96.36%   -0.04%     
- Complexity     5813     5817       +4     
============================================
  Files           634      634              
  Lines         41760    41760              
  Branches       5796     5796              
============================================
- Hits          40256    40240      -16     
- Misses          583      594      +11     
- Partials        921      926       +5

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/plugins/RxJavaPlugins.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	89.23% <0%> (-5.83%)	59% <0%> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	89.37% <0%> (-3.39%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...ain/java/io/reactivex/subjects/PublishSubject.java	97.8% <0%> (-2.2%)	38% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	84.53% <0%> (-1.65%)	3% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	93.44% <0%> (-1.64%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	97.56% <0%> (-1.63%)	3% <0%> (ø)
... and 26 more

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