2.x Fix NPE when debouncing empty source

[hluhovskyi]

Background Related issue #6558
Cherry pick of #6559 for 2.x version

Debounce with selector on the empty Observable leads to NPE.

It happens because of DebounceObserver#debouncer field is set up only when onNext is called. When onNext isn't called at all, like when debouncing Observable.empty(), we get a null reference from debouncer and call emit on null which actually leads to NPE.

Change
Wrap emit call with null check both in ObservableDebounce and FlowableDebounce

Test plan
To check Observable:

./gradlew test --tests "io.reactivex.internal.operators.observable.ObservableDebounceTest.debounceOnEmpty"`

To check Flowable:

./gradlew test --tests "io.reactivex.internal.operators.flowable.FlowableDebounceTest.debounceOnEmpty"`

[codecov]

Codecov Report

Merging #6560 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6560      +/-   ##
============================================
- Coverage     98.25%   98.24%   -0.02%     
- Complexity     6334     6337       +3     
============================================
  Files           677      677              
  Lines         45486    45488       +2     
  Branches       6319     6321       +2     
============================================
- Hits          44691    44688       -3     
- Misses          249      252       +3     
- Partials        546      548       +2

Impacted Files	Coverage Δ	Complexity Δ
.../internal/operators/flowable/FlowableDebounce.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/observable/ObservableDebounce.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-5.8%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.99%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	96.65% <0%> (-1.68%)	13% <0%> (ø)
... and 21 more

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