Add missing onSubscribe null-checks to NPE docs on Flowable/Observable subscribe

[ZacSweers]

Happened to notice these today

[codecov]

Codecov Report

Merging #6301 into 2.x will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #6301      +/-   ##
===========================================
+ Coverage     98.24%   98.3%   +0.06%     
- Complexity     6281    6284       +3     
===========================================
  Files           672     672              
  Lines         44980   44980              
  Branches       6219    6219              
===========================================
+ Hits          44189   44216      +27     
+ Misses          251     235      -16     
+ Partials        540     529      -11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	94.69% <0%> (-3.54%)	11% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	89.47% <0%> (-0.79%)	4% <0%> (ø)
... and 27 more

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[vanniktech]

(I think there are actually a lot more who are missing / inconsistent)