2.x: add nullable annotation to simple queue (fixes #5053)

[jschneider]

As explained in #5053 SimpleQueue might return null.
To document this behavior a nullable annotation has been added.

[codecov]

Codecov Report

Merging #5054 into 2.x will increase coverage by 0.01%.

@@             Coverage Diff              @@
##                2.x    #5054      +/-   ##
============================================
+ Coverage     95.65%   95.67%   +0.01%     
+ Complexity     5546     5543       -3     
============================================
  Files           612      612              
  Lines         39576    39576              
  Branches       5553     5553              
============================================
+ Hits          37857    37863       +6     
- Misses          737      744       +7     
+ Partials        982      969      -13

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/observable/ObservableFromArray.java	100% <ø> (ø)	3 <ø> (ø)	❌
...rnal/operators/observable/ObservableRangeLong.java	100% <ø> (ø)	2 <ø> (ø)	❌
...perators/maybe/MaybeFlatMapIterableObservable.java	98.66% <ø> (ø)	2 <ø> (ø)	❌
...vex/internal/operators/flowable/FlowableRange.java	96.9% <ø> (ø)	3 <ø> (ø)	❌
...x/internal/operators/flowable/FlowableGroupBy.java	95.01% <ø> (-0.63%)	2 <ø> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <ø> (ø)	28 <ø> (ø)	❌
...eactivex/internal/disposables/EmptyDisposable.java	100% <ø> (ø)	17 <ø> (ø)	❌
...nal/operators/observable/ObservableScalarXMap.java	96.55% <ø> (ø)	7 <ø> (ø)	❌
...internal/operators/flowable/FlowableObserveOn.java	97.1% <ø> (+1.15%)	3 <ø> (ø)	❌
...internal/operators/flowable/FlowableRangeLong.java	91.75% <ø> (ø)	3 <ø> (ø)	❌
... and 81 more

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