2.x: Fix concatMap{Single|Maybe} null emission on dispose race

[akarnokd]

This PR fixes a bug in all 4 specialized concatMap implementation that allows null to be emitted when the success signal of the inner source races with the dispose signal of the sequence.

Likely fixes: #6059

[akarnokd]

A very similar thing happened in #5927 just after a release but this bug was there between 2.1.11 and 2.1.14.

[codecov]

Codecov Report

Merging #6060 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6060      +/-   ##
============================================
+ Coverage     98.26%   98.29%   +0.02%     
- Complexity     6188     6194       +6     
============================================
  Files           666      666              
  Lines         44802    44806       +4     
  Branches       6206     6206              
============================================
+ Hits          44025    44041      +16     
+ Misses          235      232       -3     
+ Partials        542      533       -9

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/mixed/FlowableConcatMapMaybe.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/mixed/FlowableConcatMapSingle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...nal/operators/mixed/ObservableConcatMapSingle.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...rnal/operators/mixed/ObservableConcatMapMaybe.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	86.92% <0%> (-9.16%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-7.47%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-2.14%)	5% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
... and 24 more

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