Fix missing nullability on Single.subscribe(BiConsumer)

[ZacSweers]

Resolves #7330

[codecov]

Codecov Report

Merging #7331 (1e42ea2) into 3.x (b476914) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7331      +/-   ##
============================================
- Coverage     99.55%   99.50%   -0.05%     
+ Complexity     6781     6780       -1     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47271    47249      -22     
- Misses           96      108      +12     
- Partials        115      125      +10     

Impacted Files	Coverage Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100.00% <ø> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.90% <0.00%> (-8.26%)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.60% <0.00%> (-4.07%)	⬇️
...vex/rxjava3/internal/operators/maybe/MaybeAmb.java	96.96% <0.00%> (-3.04%)	⬇️
...java3/internal/schedulers/TrampolineScheduler.java	97.40% <0.00%> (-2.60%)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <0.00%> (-2.09%)	⬇️
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	⬇️
... and 15 more

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