Add nullPointerException comments and ObjectHelper test code.

[ggikko]

:)

[codecov]

Codecov Report

Merging #5255 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5255      +/-   ##
============================================
+ Coverage     95.97%   95.98%   +<.01%     
- Complexity     5741     5747       +6     
============================================
  Files           628      628              
  Lines         41075    41075              
  Branches       5698     5698              
============================================
+ Hits          39420    39424       +4     
+ Misses          660      655       -5     
- Partials        995      996       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...ors/observable/ObservableSampleWithObservable.java	92.77% <0%> (-6.03%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	91.17% <0%> (-4.42%)	3% <0%> (ø)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...activex/internal/subscribers/FutureSubscriber.java	96.82% <0%> (-3.18%)	32% <0%> (-1%)
...al/operators/observable/ObservableSampleTimed.java	95% <0%> (-1.67%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.05% <0%> (-1.65%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	93.28% <0%> (-1.5%)	2% <0%> (ø)
... and 33 more

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