2.x: Fix parallel() on grouped flowable not replenishing properly

[akarnokd]

Backport of #6719

Fix a case when the GroupedFlowable is consumed by a parallel() in fusion mode causing the source to stop replenishing items from the upstream, hanging the whole sequence.

parallel() was slightly different from the usual queue consumers because it checks for isEmpty before trying to pull for an item. This was necessary because the rails may not be ready for more and an eager pull to check for emptyness would lose that item. The replenishing was done in GroupedFlowable.pull but a call to GroupedFlowable.isEmpty would not replenish.

The fix is to have isEmpty replenish similar to when poll detects emptyness and replenishes.

Reported in reactor/reactor-core#1959

[codecov]

Codecov Report

Merging #6720 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6720      +/-   ##
============================================
- Coverage     98.26%   98.24%   -0.02%     
+ Complexity     6346     6345       -1     
============================================
  Files           677      677              
  Lines         45527    45532       +5     
  Branches       6333     6334       +1     
============================================
- Hits          44737    44735       -2     
- Misses          246      249       +3     
- Partials        544      548       +4

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableGroupBy.java	96.12% <100%> (+0.05%)	3 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	91.04% <0%> (-3.74%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-3.27%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-3.27%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...perators/mixed/ObservableConcatMapCompletable.java	98.49% <0%> (-1.51%)	3% <0%> (ø)
... and 22 more

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