Deleted unnecessary NPE tests

[slisaasquatch]

Resolves #6830

I have kept tests that arbitrarily throw NPEs like this one, and tests with Iterables that return null Iterators like this one.

[akarnokd]

Many null tests were deleted that should have been kept.

[codecov]

Codecov Report

Merging #6846 into 3.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6846      +/-   ##
============================================
- Coverage     98.22%   98.21%   -0.02%     
+ Complexity     6380     6379       -1     
============================================
  Files           715      715              
  Lines         46522    46522              
  Branches       6387     6387              
============================================
- Hits          45698    45692       -6     
+ Misses          306      299       -7     
- Partials        518      531      +13

Impacted Files	Coverage Δ	Complexity Δ
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	88.88% <0%> (-3.18%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	86.61% <0%> (-2.82%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.66% <0%> (-2.09%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98.18% <0%> (-1.82%)	3% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0%> (-1.31%)	3% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.07%)	5% <0%> (ø)
...java3/internal/operators/flowable/FlowableZip.java	97.39% <0%> (-1.05%)	6% <0%> (ø)
... and 8 more

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