2.x: add missing null check to fused Observable.fromCallable

[akarnokd]

There was a missing null check on the fusion path in ObservableFromCallable which meant the consumer considered the source to be empty. (The FlowableFromCallable was okay.)

Discovered on StackOverflow: https://stackoverflow.com/questions/45304226/null-handling-in-rx-java2-flatmap

Unit test mirrored and updated to verify both Flowable and Observable versions.

[codecov]

Codecov Report

Merging #5517 into 2.x will decrease coverage by 0.16%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5517      +/-   ##
============================================
- Coverage     96.24%   96.07%   -0.17%     
+ Complexity     5811     5801      -10     
============================================
  Files           631      631              
  Lines         41285    41285              
  Branches       5732     5732              
============================================
- Hits          39734    39665      -69     
- Misses          604      637      +33     
- Partials        947      983      +36

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFromCallable.java	100% <100%> (ø)	5 <1> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-6.72%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.72% <0%> (-6.64%)	61% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-6.56%)	2% <0%> (ø)
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
...internal/disposables/ArrayCompositeDisposable.java	96.42% <0%> (-3.58%)	15% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.75% <0%> (-3.39%)	2% <0%> (ø)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	95.58% <0%> (-2.95%)	2% <0%> (ø)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	86.27% <0%> (-2.62%)	2% <0%> (ø)
... and 39 more

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