Fix missing NonNull on Observable.map

[ZacSweers]

I noticed this while testing the upcoming Kotlin 1.5.30-M1 (which has improved support for type-use nullability annotations) https://youtrack.jetbrains.com/issue/KT-47833. This makes at least match Single, but Maybe.map is also missing this. I suspect there are others too.

[codecov]

Codecov Report

Merging #7302 (677eafe) into 3.x (bb814d2) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7302      +/-   ##
============================================
+ Coverage     99.53%   99.54%   +0.01%     
+ Complexity     6783     6781       -2     
============================================
  Files           751      751              
  Lines         47489    47489              
  Branches       6378     6378              
============================================
+ Hits          47266    47271       +5     
+ Misses          102      100       -2     
+ Partials        121      118       -3     

Impacted Files	Coverage Δ
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	93.66% <0.00%> (-4.23%)	⬇️
...x/rxjava3/internal/operators/single/SingleAmb.java	96.61% <0.00%> (-3.39%)	⬇️
...3/internal/operators/flowable/FlowablePublish.java	98.01% <0.00%> (-1.99%)	⬇️
...nternal/operators/observable/ObservableWindow.java	98.21% <0.00%> (-1.79%)	⬇️
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.60% <0.00%> (-1.17%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.37% <0.00%> (-1.04%)	⬇️
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	⬇️
...operators/flowable/FlowableConcatMapScheduler.java	99.20% <0.00%> (-0.40%)	⬇️
...ternal/operators/observable/ObservableFlatMap.java	98.22% <0.00%> (ø)
... and 9 more

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[akarnokd]

FYI, I've sweeped across the code base and clarified the nullability annotations on generic type arguments in #7303.