Add NullPointerException comment

[ggikko]

Add comments in Single.java

[codecov]

Codecov Report

Merging #5578 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5578      +/-   ##
============================================
+ Coverage     96.18%   96.22%   +0.03%     
+ Complexity     5824     5817       -7     
============================================
  Files           631      631              
  Lines         41421    41421              
  Branches       5739     5739              
============================================
+ Hits          39841    39857      +16     
+ Misses          636      611      -25     
- Partials        944      953       +9

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	132 <0> (ø)	⬇️
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-5.23%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.75% <0%> (-2.18%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
... and 29 more

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