Added nullable annotations to subjects

[bangarharshit]

Adding nullable annotations to subjects. The throwable and getValue methods are annotated with Nullable.

Quick question about styling - In some places, javadocs has trailing whitespaces but it is absent in other places. Is there a styling policy around that? Intellij is removing the trailed whitespace automatically.

[codecov]

Codecov Report

Merging #5890 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5890      +/-   ##
===========================================
- Coverage     97.63%   97.6%   -0.03%     
- Complexity     5947    5948       +1     
===========================================
  Files           655     655              
  Lines         43860   43860              
  Branches       6098    6098              
===========================================
- Hits          42821   42811      -10     
- Misses          325     328       +3     
- Partials        714     721       +7

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/subjects/AsyncSubject.java	99.06% <ø> (ø)	52 <0> (ø)	⬇️
.../java/io/reactivex/subjects/SerializedSubject.java	95.78% <ø> (ø)	27 <0> (ø)	⬇️
...java/io/reactivex/subjects/CompletableSubject.java	100% <ø> (ø)	36 <0> (ø)	⬇️
...ain/java/io/reactivex/subjects/PublishSubject.java	100% <ø> (ø)	39 <0> (ø)	⬇️
.../main/java/io/reactivex/subjects/MaybeSubject.java	100% <ø> (ø)	46 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	96.23% <ø> (ø)	54 <0> (ø)	⬇️
...ain/java/io/reactivex/subjects/UnicastSubject.java	100% <ø> (ø)	65 <0> (ø)	⬇️
...main/java/io/reactivex/subjects/ReplaySubject.java	97.56% <ø> (ø)	49 <0> (ø)	⬇️
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-2.9%)	2% <0%> (ø)
... and 19 more

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[vanniktech]

Trailing whitespace was added by accident and should not be there so it's fine to remove it.