Fixed defaultIfEmpty incorrect type with strictNullChecks off

[shessafridi]

Description:

defaultIfEmpty(null) or defaultIfEmpty(undefined) always returns the observable of type Observable<any>

Expected behavior

When strictNullChecks is off null type is ignored so it should just return the type of the source observable as it is.

Expected behaviour in strictNullChecks: false

const test$ = of(1).pipe(defaultIfEmpty(null)); // Observable<number>

Instead we get

const test$ = of(1).pipe(defaultIfEmpty(null)); // Observable<any>

This PR fixes the issue by adding explicit overloads for null and undefined

Related issue (if exists): #7280

[shessafridi]

ping @benlesh

[shessafridi]

ping @jakovljevic-mladen

[conblem]

What happenened to this pr?

[benlesh]

Honestly, I remember glancing at it, and thinking about it, but deciding (at the time) that it doesn't seem like something we want to support. If people want to turn off strict settings in TypeScript, that's their business. I left it here because I wasn't totally convinced I was correct, but every time I revisited it, I had the same thought. That it wasn't the right thing to do.

Sorry @shessafridi I should have more openly written out my thoughts. Thank you for your contribution.