Fix #508 FlattenIterableSubscriber.isEmpty check

[lhotari]

• returned true from isEmpty when iterator had more elements

[lhotari]

It seems that there was no existing test coverage for the code path. Perhaps we should add some more tests? Any guidance for tests that I could add?

[codecov-io]

Codecov Report

Merging #509 into master will increase coverage by 0.02%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #509      +/-   ##
============================================
+ Coverage     78.66%   78.69%   +0.02%     
- Complexity     2826     2828       +2     
============================================
  Files           274      274              
  Lines         24474    24474              
  Branches       4201     4202       +1     
============================================
+ Hits          19253    19260       +7     
- Misses         3794     3796       +2     
+ Partials       1427     1418       -9

Impacted Files	Coverage Δ	Complexity Δ
...va/reactor/core/publisher/FluxFlattenIterable.java	83.22% <0%> (+0.31%)	6 <0> (ø)	⬇️
...a/reactor/core/publisher/FluxSubscribeOnValue.java	60.82% <0%> (-2.07%)	3% <0%> (ø)
.../java/reactor/core/scheduler/ElasticScheduler.java	74.01% <0%> (-1.97%)	21% <0%> (-2%)
...eactor/core/publisher/ParallelMergeSequential.java	75.25% <0%> (-0.51%)	3% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxCreate.java	81.04% <0%> (-0.28%)	7% <0%> (ø)
...rc/main/java/reactor/core/publisher/Operators.java	59.89% <0%> (+0.17%)	66% <0%> (ø)	⬇️
...java/reactor/core/scheduler/ParallelScheduler.java	70.98% <0%> (+0.51%)	19% <0%> (ø)	⬇️
...n/java/reactor/core/publisher/DirectProcessor.java	82.9% <0%> (+0.85%)	34% <0%> (+1%)	⬆️
.../java/reactor/core/publisher/EmitterProcessor.java	85.15% <0%> (+1.06%)	105% <0%> (+3%)	⬆️
...actor/core/scheduler/ExecutorServiceScheduler.java	81.41% <0%> (+1.28%)	16% <0%> (ø)	⬇️
... and 2 more

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[lbovet]

Hey Lari, looks like you found the April Fool Day bug! You just add a "!" and solve our day-long-mind-blogging-mystery. Many thanks for your reactiv ity!

[smaldini]

Wow thanks a lot @lhotari !