fix(@angular-devkit/core): host.delete() completes but never emits

[alan-agius4]

RxJS EMPTY creates an Observable that emits no items immediately emits a complete notification.

Fixes #14235

[filipesilva]

Ran into problems with these in the past too, thought we had fixed it already.

I'm not sure what the right thing to put in the of() is if we don't care about it, but there's only one of(undefined) and one of(true) in core, but a bunch of of(null). So maybe of(null) is a better pattern if only for consistency.

[filipesilva]

Discussed offline, of(null) can't be used because the expected return type is Observable<void>.

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