fix(cdk/scrolling): expand type for "cdkVirtualForOf" input to… (#17421)

Currently the `cdkVirtualForOf` 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 definition of the input becomes too strict with "--strictNullChecks" enabled.

For extra content: by default if strict null checks are not enabled, `null` or `undefined`
are assignable to _every_ type. The current type definiton of the `cdkVirtualForOf` input
is incorrectly making the assumption that `null` or `undefined` are always assignable.

The type of the input needs to be expanded to explicitly accept `null` or
`undefined` to behave consistently regardless of the `strictNullChecks` flag.

A common scenario where this breaks is the use of `cdkVirtualFor` in combination
with the async pipe from `@angular/common`. This is because the async pipe returns
`null` until a value has been emitted. This means that with strict null checks, the type
checking will fail because `null` is not allowed.

This is similar to what we did for `ngFor` in framework: angular/angular@c1bb886

Fixes #17411

src/cdk/scrolling/virtual-for-of.ts

@@ -83,18 +83,18 @@ export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy
 
   /** The DataSource to display. */
   @Input()
-  get cdkVirtualForOf(): DataSource<T> | Observable<T[]> | NgIterable<T> {
+  get cdkVirtualForOf(): DataSource<T> | Observable<T[]> | NgIterable<T> | null | undefined {
     return this._cdkVirtualForOf;
   }
-  set cdkVirtualForOf(value: DataSource<T> | Observable<T[]> | NgIterable<T>) {
+  set cdkVirtualForOf(value: DataSource<T> | Observable<T[]> | NgIterable<T> | null | undefined) {
     this._cdkVirtualForOf = value;
     const ds = isDataSource(value) ? value :
         // Slice the value if its an NgIterable to ensure we're working with an array.
         new ArrayDataSource<T>(
             value instanceof Observable ? value : Array.prototype.slice.call(value || []));
     this._dataSourceChanges.next(ds);
   }
-  _cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T>;
+  _cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T> | null | undefined;
 
   /**
    * The `TrackByFunction` to use for tracking changes. The `TrackByFunction` takes the index and
@@ -363,7 +363,9 @@ export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy
     // comment node which can throw off the move when it's being repeated for all items.
     return this._viewContainerRef.createEmbeddedView(this._template, {
       $implicit: null!,
-      cdkVirtualForOf: this._cdkVirtualForOf,
+      // It's guaranteed that the iterable is not "undefined" or "null" because we only
+      // generate views for elements if the "cdkVirtualForOf" iterable has elements.
+      cdkVirtualForOf: this._cdkVirtualForOf!,
       index: -1,
       count: -1,
       first: false,

tools/public_api_guard/cdk/scrolling.d.ts

@@ -60,8 +60,8 @@ export declare class CdkScrollable implements OnInit, OnDestroy {
 }
 
 export declare class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy {
-    _cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T>;
-    cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T>;
+    _cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T> | null | undefined;
+    cdkVirtualForOf: DataSource<T> | Observable<T[]> | NgIterable<T> | null | undefined;
     cdkVirtualForTemplate: TemplateRef<CdkVirtualForOfContext<T>>;
     cdkVirtualForTemplateCacheSize: number;
     cdkVirtualForTrackBy: TrackByFunction<T> | undefined;