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

packages/common/src/directives/ng_for_of.ts

@@ -129,7 +129,7 @@ export class NgForOf<T> implements DoCheck {
    * [template input variable](guide/structural-directives#template-input-variable).
    */
   @Input()
-  set ngForOf(ngForOf: NgIterable<T>) {
+  set ngForOf(ngForOf: NgIterable<T>|undefined|null) {
     this._ngForOf = ngForOf;
     this._ngForOfDirty = true;
   }
@@ -165,8 +165,7 @@ export class NgForOf<T> implements DoCheck {
 
   get ngForTrackBy(): TrackByFunction<T> { return this._trackByFn; }
 
-  // TODO(issue/24571): remove '!'.
+  private _ngForOf: NgIterable<T>|undefined|null = null;
-  private _ngForOf !: NgIterable<T>;
   private _ngForOfDirty: boolean = true;
   private _differ: IterableDiffer<T>|null = null;
   // TODO(issue/24571): remove '!'.
@@ -219,8 +218,11 @@ export class NgForOf<T> implements DoCheck {
         (item: IterableChangeRecord<any>, adjustedPreviousIndex: number | null,
          currentIndex: number | null) => {
           if (item.previousIndex == null) {
+            // NgForOf is never "null" or "undefined" here because the differ detected
+            // that a new item needs to be inserted from the iterable. This implies that
+            // there is an iterable value for "_ngForOf".
             const view = this._viewContainer.createEmbeddedView(
-                this._template, new NgForOfContext<T>(null !, this._ngForOf, -1, -1),
+                this._template, new NgForOfContext<T>(null !, this._ngForOf !, -1, -1),
                 currentIndex === null ? undefined : currentIndex);
             const tuple = new RecordViewTuple<T>(item, view);
             insertTuples.push(tuple);
@@ -243,7 +245,7 @@ export class NgForOf<T> implements DoCheck {
       const viewRef = <EmbeddedViewRef<NgForOfContext<T>>>this._viewContainer.get(i);
       viewRef.context.index = i;
       viewRef.context.count = ilen;
-      viewRef.context.ngForOf = this._ngForOf;
+      viewRef.context.ngForOf = this._ngForOf !;
     }
 
     changes.forEachIdentityChange((record: any) => {

packages/core/src/change_detection/differs/default_iterable_differ.ts

@@ -146,7 +146,7 @@ export class DefaultIterableDiffer<V> implements IterableDiffer<V>, IterableChan
     }
   }
 
-  diff(collection: NgIterable<V>): DefaultIterableDiffer<V>|null {
+  diff(collection: NgIterable<V>|null|undefined): DefaultIterableDiffer<V>|null {
     if (collection == null) collection = [];
     if (!isListLikeIterable(collection)) {
       throw new Error(

packages/core/src/change_detection/differs/iterable_differs.ts

@@ -34,7 +34,7 @@ export interface IterableDiffer<V> {
    * @returns an object describing the difference. The return value is only valid until the next
    * `diff()` invocation.
    */
-  diff(object: NgIterable<V>): IterableChanges<V>|null;
+  diff(object: NgIterable<V>|undefined|null): IterableChanges<V>|null;
 }
 
 /**

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);

tools/public_api_guard/core/core.d.ts

@@ -263,7 +263,7 @@ export declare class DefaultIterableDiffer<V> implements IterableDiffer<V>, Iter
     readonly length: number;
     constructor(trackByFn?: TrackByFunction<V>);
     check(collection: NgIterable<V>): boolean;
-    diff(collection: NgIterable<V>): DefaultIterableDiffer<V> | null;
+    diff(collection: NgIterable<V> | null | undefined): DefaultIterableDiffer<V> | null;
     forEachAddedItem(fn: (record: IterableChangeRecord_<V>) => void): void;
     forEachIdentityChange(fn: (record: IterableChangeRecord_<V>) => void): void;
     forEachItem(fn: (record: IterableChangeRecord_<V>) => void): void;
@@ -505,7 +505,7 @@ export interface IterableChanges<V> {
 }
 
 export interface IterableDiffer<V> {
-    diff(object: NgIterable<V>): IterableChanges<V> | null;
+    diff(object: NgIterable<V> | undefined | null): IterableChanges<V> | null;
 }
 
 export interface IterableDifferFactory {