3.x: XProcessor.offer to throw NPE immediately

[akarnokd]

Make offer throw a NullPointerException immediately instead of turning it into an error signal. In addition, have MulticastProcessor.offer throw an IllegalStateException if called when the processor is in fusion mode.

Resolves #6794

[codecov]

Codecov Report

Merging #6799 into 3.x will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6799      +/-   ##
============================================
- Coverage     98.21%   98.17%   -0.05%     
  Complexity     6375     6375              
============================================
  Files           711      711              
  Lines         46217    46214       -3     
  Branches       6329     6330       +1     
============================================
- Hits          45393    45370      -23     
- Misses          298      309      +11     
- Partials        526      535       +9

Impacted Files	Coverage Δ	Complexity Δ
...eactivex/rxjava3/processors/BehaviorProcessor.java	98.02% <100%> (+1.44%)	49 <4> (-2)	⬇️
...reactivex/rxjava3/processors/PublishProcessor.java	100% <100%> (ø)	42 <4> (-1)	⬇️
...activex/rxjava3/processors/MulticastProcessor.java	98.68% <100%> (+1.32%)	84 <4> (+1)	⬆️
...nternal/operators/observable/ObservableCreate.java	79.48% <0%> (-19.66%)	2% <0%> (ø)
...observable/ObservableWindowSubscribeIntercept.java	87.5% <0%> (-12.5%)	4% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	92.85% <0%> (-3.18%)	2% <0%> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.81% <0%> (-2.73%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.14% <0%> (-2.61%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98.18% <0%> (-1.82%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-1.75%)	2% <0%> (ø)
... and 18 more

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