Remove the @NonNull annotation from Function

[ansman]

As per per the discussion in #5447

[codecov]

Codecov Report

Merging #5449 into 2.x will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5449      +/-   ##
===========================================
+ Coverage     95.92%     96%   +0.07%     
+ Complexity     5797    5795       -2     
===========================================
  Files           631     631              
  Lines         41306   41306              
  Branches       5745    5745              
===========================================
+ Hits          39624   39656      +32     
+ Misses          664     640      -24     
+ Partials       1018    1010       -8

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/flowable/FlowableSubscribeOn.java	91.52% <0%> (-6.78%)	2% <0%> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <0%> (-4.09%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	91.3% <0%> (-3.63%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <0%> (-2.39%)	38% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...activex/internal/schedulers/ExecutorScheduler.java	94.59% <0%> (-2.03%)	9% <0%> (ø)
... and 40 more

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