2.x: fix flatMapIterable appearing to be empty when fused

[akarnokd]

The wrong logical expression in the isEmpty() method made flatMapIterable complete earlier, even if data was available in the current iterable.

Somewhat related: reactor/reactor-core#508 (different logical bug in the same method).

[codecov]

Codecov Report

Merging #5256 into 2.x will increase coverage by 0.04%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##                2.x    #5256      +/-   ##
============================================
+ Coverage     95.95%   95.99%   +0.04%     
+ Complexity     5750     5746       -4     
============================================
  Files           628      628              
  Lines         41075    41077       +2     
  Branches       5698     5699       +1     
============================================
+ Hits          39414    39433      +19     
+ Misses          664      657       -7     
+ Partials        997      987      -10

Impacted Files	Coverage Δ	Complexity Δ
...al/operators/flowable/FlowableFlattenIterable.java	97.24% <66.66%> (+0.02%)	4 <0> (ø)	⬇️
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <0%> (-6.67%)	17% <0%> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-4.39%)	9% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.13% <0%> (-3.27%)	9% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <0%> (-2.39%)	38% <0%> (-1%)
... and 39 more

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