Modified behavior on empty inputs for forkJoin, combineLatest and zip

[rraziel]

Description:

The forkJoin, combineLatest and zip functions have been modified so that:

combineLatest();   // Observable<never>: behavior kept for compatibility as the non-array version is deprecated
combineLatest([]); // Observable<[]>: now emits an empty array before completing
combineLatest({}); // Observable<{}>: now emits an empty object before completing

forkJoin();   // Observable<never>: behavior kept for compatibility as the non-array version is deprecated
forkJoin([]); // Observable<[]>: now emits an empty array before completing
forkJoin({}); // Observable<{}>, now emits an empty object before completing

zip();     // Observable<never>, behavior kept for compatibility as the non-array version is now deprecated
zip(a,b)   // /!\ has been marked as deprecated in this PR /!\
zip([]);   // Observable<[]>: now emits an empty array before completing
zip({a,b}) // Observable<{a: A, b: B}>: new behavior to align with the other ones
           // /!\ new behavior to align with the other ones /!\
zip({});   // Observable<{}>: emits an empty object then completes

// side effects
zipAll() // now emits [] for empty observables

Related issue (if exists):

Regarding the signature (#5066), in particular needs N-args signature for observable inputs:
All 3 functions seem to have the correct <A extends readonly unknown[]>(sources: [...ObservableInputTuple<A>]): Observable<A>

Closes #5209

[rraziel]

Note: if the deprecated signatures can be removed because of the major version bump (except for the ones I just added on zip I suppose), I can update the PR; let me know

[rraziel]

@benlesh The fix in 7.0.0-beta.10 contradicts this PR and #5209 's comments, should this PR be dropped?

[benlesh]

I really appreciate the effort that went into here, but I don't think we can accept this PR.

With these creation functions, the supplied object or array represents a dictionary or list of observable sources, respectively. If those are empty ({} or []), then there are no observables to subscribe to, and therefor, nothing to emit values to emit as an array of values or a dictionary of values... So the behavior should be that it emits an EMPTY which is Observable<never>.

I hope that makes sense.

Thanks again for your work!

[rraziel]

@benlesh No worries, I never got an answer on the comments from #5209 so it's not like anyone told me "go for it" (would have been nice to get a "don't go for it" though 😅)
Should I still create a PR for the zip({}) code that was also in there (same as forkJoin({}) and combineLatest({}))?