Remove @NonNull annotations in BiConsumer method parameter

[bloderxd]

This PR is part of #5216

This PR remove @nonnull annotations from BiConsumer method parameters, there's no sense use these annotations in Single context because in this case BiConsumer always brings you a null parameter.

[codecov]

Codecov Report

Merging #5257 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5257      +/-   ##
============================================
- Coverage     95.97%   95.94%   -0.03%     
+ Complexity     5751     5743       -8     
============================================
  Files           628      628              
  Lines         41077    41077              
  Branches       5699     5699              
============================================
- Hits          39422    39412      -10     
- Misses          664      668       +4     
- Partials        991      997       +6

Impacted Files	Coverage Δ	Complexity Δ
...al/operators/observable/ObservableSampleTimed.java	90% <0%> (-8.34%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	91.52% <0%> (-6.78%)	2% <0%> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	89.7% <0%> (-5.89%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-5.23%)	2% <0%> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	95.23% <0%> (-4.77%)	4% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...activex/internal/operators/single/SingleUsing.java	97.59% <0%> (-2.41%)	4% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
... and 27 more

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[akarnokd]

/cc @vanniktech

I belive BiConsumer is very often appears with zip and combineLatest where the @NonNull annotation is important.

[JakeWharton]

It's not that important. It just means nullability contracts need to be defined where BiConsumer is used instead of on the interface itself.

[bloderxd]

Well, it's not in all context that @nonnull annotation is correct in BiConsumer, then I guess it's not safe let it there

[vanniktech]

Might make more sense to delete the annotations since the interface is used in different places where different parameters are sometimes null and sometimes not.

Having different interfaces with different annotations tailored for each use case might be the best option although it wouldn't be backwards compatible at least not source and binary compatible.

[bloderxd]

@vanniktech like this example?

 public void biconsumerWithCorrectNullableAnnotationInThrowable() {
        Single.just(1).subscribe(new SingleBiConsumer<Integer, Throwable>() {
            @Override
            public void accept(@Nullable Integer integer, @Nullable Throwable throwable) throws Exception {
                assertNull(throwable);
            }
        });
    }

Create a extesion of BiConsumer only for Single Context?