3.x: Fix map() conditional chain causing NPE

[akarnokd]

Fix the case when an (async) fused filter-map-filter chain does not handle the null indicator in its conditional path inside map.

Fixes #7039

[loganvolkers]

❤️ wow so happy with the turnaround on this!

[codecov]

Codecov Report

Merging #7040 into 3.x will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7040      +/-   ##
============================================
- Coverage     99.54%   99.50%   -0.05%     
+ Complexity     6671     6668       -3     
============================================
  Files           742      742              
  Lines         47270    47273       +3     
  Branches       6373     6374       +1     
============================================
- Hits          47057    47038      -19     
- Misses           97      110      +13     
- Partials        116      125       +9     

Impacted Files	Coverage Δ	Complexity Δ
...java3/internal/operators/flowable/FlowableMap.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.90% <0.00%> (-3.67%)	2.00% <0.00%> (ø%)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...l/operators/observable/ObservableFlatMapMaybe.java	92.25% <0.00%> (-2.82%)	2.00% <0.00%> (ø%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0.00%> (-2.78%)	56.00% <0.00%> (-2.00%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
.../internal/disposables/ListCompositeDisposable.java	98.00% <0.00%> (-2.00%)	34.00% <0.00%> (-1.00%)
...nternal/operators/parallel/ParallelSortedJoin.java	98.49% <0.00%> (-1.51%)	2.00% <0.00%> (ø%)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0.00%> (-1.31%)	3.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFromIterable.java	98.95% <0.00%> (-1.05%)	5.00% <0.00%> (ø%)
... and 11 more

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