fix(takeUntil): takeUntil should subscribe to the source if notifier synchronously completes without emitting

[jayphelps]

This bug was introduced in #3504, which intended to not subscribe to the source if the notifier synchronously emits a value, but instead it only skipped subscribe if it completed

https://stackblitz.com/edit/js-j72dzx?file=index.js

import { of, empty } from 'rxjs';
import { takeUntil } from 'rxjs/operators';

// does not log anything, but it should
of(1, 2, 3).pipe(
  takeUntil(empty())
).subscribe(d => console.log(d));

---

Like most bugfixes this could potentially be a breaking change if someone is knowingly or unknowingly relying on this fact. Dunno what the current policy is on breaking bug fixes.

[coveralls]

Coverage increased (+0.002%) to 96.949% when pulling 44e6878 on jayphelps:takeUntil into ef3ff0b on ReactiveX:master.

[cartant]

LGTM. In the docs, some notifiers - e.g. for delayWhen - are described as effecting a particular behaviour when the notifier completes, but that isn't the case for the takeUntil docs. They are explicit in stating that the notifier has to emit a value, so I think this fix should be merged.