fix(push): fix return typing for observables to include undefined (#2907)

BREAKING CHANGE:

BEFORE:

ngrxPush typing doesn't consider `undefined` when the input type is an observable

AFTER:

ngrxPush typing considers `undefined` when the input type is an observable

Closes #2888

Author: samuelfernandez

modules/component/src/push/push.pipe.ts

@@ -79,7 +79,7 @@ export class PushPipe<S> implements PipeTransform, OnDestroy {
 
   transform<T>(potentialObservable: null): null;
   transform<T>(potentialObservable: undefined): undefined;
-  transform<T>(potentialObservable: ObservableInput<T>): T;
+  transform<T>(potentialObservable: ObservableInput<T>): T | undefined;
   transform<T>(
     potentialObservable: ObservableInput<T> | null | undefined
   ): T | null | undefined {