2.x: ElementAt Operators in Observable and Flowable don't behave correctly on empty sources

[vanniktech]

elementAtOrError(1) on an empty source just leaves the new Single hanging without signalling onError():

Observable.empty()
    .elementAtOrError(1)
    .test()
    .assertFailure(NoSuchElementException.class);

elementAt(1) on an empty source just leaves the Maybe hanging without signalling onComplete():

Observable.empty()
    .elementAt(1)
    .test()
    .assertResult();

Just tested this against RC4. Happy to fix this.

[akarnokd]

Sure.

[akarnokd]

Closing via #4681