fix(common): expand type for "ngForOf" input to work with strict null checks (#31371)

Currently the `ngForOf` input accepts `null` or `undefined` as valid
values. Although when using strict template input type checking
(which will be supported by `ngtsc`), passing `null` or `undefined`
with strict null checks enabled causes a type check failure because
the type for the `ngForOf` input becomes too strict if strict null checks
are enabled. The type of the input needs to be expanded to also accept
`null` or `undefined` to behave consistently regardless of the
`strictNullChecks` flag.

This is necessary because whenever strict input type checking is enabled
by default, most of the Angular projects that use `*ngFor` with the async pipe
will either need to disable template type checking or strict null checks
because the `async` pipe returns `null` if the observable hasn't been
emitted yet.

See for example how this affects the `angular/components` repository and
how much bloat the workaround involves: https://github.com/angular/components/pull/16373/files#r296942696.

PR Close #31371

tools/public_api_guard/common/common.d.ts

@@ -239,7 +239,7 @@ export declare class NgComponentOutlet implements OnChanges, OnDestroy {
 }
 
 export declare class NgForOf<T> implements DoCheck {
-    ngForOf: NgIterable<T>;
+    ngForOf: NgIterable<T> | undefined | null;
     ngForTemplate: TemplateRef<NgForOfContext<T>>;
     ngForTrackBy: TrackByFunction<T>;
     constructor(_viewContainer: ViewContainerRef, _template: TemplateRef<NgForOfContext<T>>, _differs: IterableDiffers);