3.x: Fix groupBy group emission in some cases

[akarnokd]

This PR fixes two issues:

• when the Observable.groupBy operator would abandon a newly created group before the group's only item was emitted, causing dataloss.
• when the groupBy's value selector failed for a new group, thus the new group would never be emitted and report the failure.

It should fix the first test failure of #6663 but may not be the cause for the second.

[codecov]

Codecov Report

Merging #6664 into 3.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6664      +/-   ##
============================================
- Coverage     98.12%   98.11%   -0.01%     
  Complexity     6188     6188              
============================================
  Files           677      677              
  Lines         44674    44682       +8     
  Branches       6166     6169       +3     
============================================
+ Hits          43835    43840       +5     
- Misses          296      299       +3     
  Partials        543      543

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableGroupBy.java	95.5% <100%> (-0.24%)	3 <0> (ø)
...ternal/operators/observable/ObservableGroupBy.java	95.62% <100%> (+0.68%)	2 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-6.35%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	95.72% <0%> (-1.71%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	94.39% <0%> (-1.41%)	3% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	96.11% <0%> (-1.3%)	6% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
... and 18 more

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