3.x Fix NPE when debouncing empty source

[hluhovskyi]

Background Related issue #6558

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 #6559 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6559      +/-   ##
============================================
- Coverage     98.16%   98.13%   -0.03%     
- Complexity     6198     6199       +1     
============================================
  Files           682      682              
  Lines         45459    45461       +2     
  Branches       6288     6290       +2     
============================================
- Hits          44626    44615      -11     
- Misses          275      288      +13     
  Partials        558      558

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)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-6.54%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-1.94%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-1.67%)	2% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	98.49% <0%> (-1.51%)	3% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b40a892...4c644de. Read the comment docs.