3.x: Fix javadoc wording of {Publish|Behavior}Processor::offer()

[akarnokd]

The Javadoc of BehaviorProcessor::offer still mentioned PublishProcessor. Also calling with null would throw an NPE and no longer error the subscribers.

[codecov]

Codecov Report

Merging #7328 (d78fc22) into 3.x (939b5ce) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7328      +/-   ##
============================================
- Coverage     99.57%   99.53%   -0.05%     
+ Complexity     6783     6779       -4     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47281    47260      -21     
- Misses           92       99       +7     
- Partials        109      123      +14     

Impacted Files	Coverage Δ
...eactivex/rxjava3/processors/BehaviorProcessor.java	100.00% <ø> (ø)
...reactivex/rxjava3/processors/PublishProcessor.java	100.00% <ø> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.74% <0.00%> (-6.43%)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-3.97%)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-3.97%)	⬇️
...internal/operators/completable/CompletableAmb.java	96.72% <0.00%> (-3.28%)	⬇️
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-2.12%)	⬇️
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	⬇️
... and 7 more

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