2.x Flowable::amb(Iterable) and Observable::amb(Iterable) throw NP

[VictorAlbertos]

I was working on #4633 and I found this behaviour, which is not consistent with the current implementation of Maybe::ambIterable, Single::ambIterable and Completable::ambIterable.

The next tests throws NP

@Test
public void Verify_Flowable_Amb_Iterable() throws InterruptedException {
  List<Flowable<String>> flowables = Arrays.asList(null, null, null);
  Flowable.amb(flowables).test();
}

@Test
public void Verify_Observable_Amb_Iterable() throws InterruptedException {
  List<Observable<String>> observables = Arrays.asList(null, null, null);
  Observable.amb(observables).test();
}

But for Maybe, Single and Completable the exception is catched and passed to the pipeline error.

Becase I'm using Flowable::amb(Iterable) and Observable::amb(Iterable) as a model to merge Maybe::ambIterable, Single::ambIterable and Completable::ambIterable into a single amb operator, I thought that Flowable::amb(Iterable) and Observable::amb(Iterable) should catch the error too. Doing something like this.

[akarnokd]

Yes, they should be signalling NPE. Since traversing the Iterable happens way before the subscription, you could just end the traversal via EmptySubscription.error(npe, s) right there and not even subscribe to any non-null source.

[VictorAlbertos]

Perfect, and I'll add some test to cover this scenario.

Should I use the same PR to address both this issue and #4633?

[akarnokd]

It's up to you; if there is no conflicting changes, you can do it separately.