3.x: Move nullness annotation to use sites of Supplier

[akarnokd]

Adds a nullability to Maybe.fromSupplier and Maybe.fromCallable to avoid warnings in Kotlin when returning nulls from the callbacks.

The previous attempt at fixing it in #7384 turned out to be not correct. Based on the responses in the Kotlin Issue report, the lack of warning was the bug and the approach was incorrect from the typesystem perspective.

Therefore, the workaround is to drop the non-null constraint from the Supplier declaration and have them at relevant use sites. (https://youtrack.jetbrains.com/issue/KT-50734#focus=Comments-27-5706908.0-0)

There was no need to add @NonNull everywhere where Supplier is used because such sites were already constrained by @NonNull in the type variable declaration.

Resolves #7376

[codecov]

Codecov Report

Merging #7385 (d819ba2) into 3.x (3ed1bfb) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7385      +/-   ##
============================================
- Coverage     99.56%   99.52%   -0.05%     
+ Complexity     6781     6780       -1     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47276    47255      -21     
- Misses           91      105      +14     
- Partials        115      122       +7     

Impacted Files	Coverage Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100.00% <ø> (ø)
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100.00% <ø> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	95.14% <0.00%> (-3.89%)	⬇️
...x/rxjava3/internal/operators/single/SingleAmb.java	96.61% <0.00%> (-3.39%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	⬇️
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-1.41%)	⬇️
... and 8 more

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