3.x: Fix many marbles in Maybe

[akarnokd]

Add many missing and replace incorrect marble diagrams in Maybe.

[codecov]

Codecov Report

Merging #6866 into 3.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6866      +/-   ##
============================================
- Coverage     98.23%   98.21%   -0.03%     
- Complexity     6379     6381       +2     
============================================
  Files           715      715              
  Lines         46528    46528              
  Branches       6388     6388              
============================================
- Hits          45709    45698      -11     
- Misses          292      307      +15     
+ Partials        527      523       -4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	180 <0> (ø)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	90.29% <0%> (-7.12%)	6% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-1.71%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	89.63% <0%> (-1.01%)	3% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
...ternal/operators/observable/ObservableGroupBy.java	95.08% <0%> (-0.55%)	2% <0%> (ø)
...activex/rxjava3/processors/MulticastProcessor.java	98.25% <0%> (-0.44%)	82% <0%> (-2%)
...3/internal/operators/flowable/FlowableGroupBy.java	95.91% <0%> (-0.32%)	3% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.28%)	3% <0%> (ø)
... and 14 more

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