fix(forms): include null in .parent of abstract control (angular#32671)

It's perfectly valid for an abstract control not to have a defined parent; yet previously the
types were asserting that AbstractControl#parent is not a null value. This changes correctly
reflects the run-time behavior through the types.

BREAKING CHANGE: Type of AbstractFormControl.parent now includes null

`null` is now included in the types of .parent. If you don't already have a check for this case,
the TypeScript compiler might compain. A v11 migration exists which adds the not-null assertion
operator where necessary.

In an unlikely case your code was testing the parnet against undefined with sitrct equality,
you'll need to change this to `=== null` instead, since the parent is not explicily initialized
with `null` instead of being left `undefined`.

Fixes angular#16999

PR Close angular#32671

Author: lazarljubenovic

goldens/public-api/forms/forms.d.ts

@@ -6,7 +6,7 @@ export declare abstract class AbstractControl {
     get enabled(): boolean;
     readonly errors: ValidationErrors | null;
     get invalid(): boolean;
-    get parent(): FormGroup | FormArray;
+    get parent(): FormGroup | FormArray | null;
     get pending(): boolean;
     readonly pristine: boolean;
     get root(): AbstractControl;

packages/forms/src/model.ts

@@ -173,8 +173,7 @@ export abstract class AbstractControl {
   // TODO(issue/24571): remove '!'.
   _updateOn!: FormHooks;
 
-  // TODO(issue/24571): remove '!'.
-  private _parent!: FormGroup|FormArray;
+  private _parent: FormGroup|FormArray|null = null;
   private _asyncValidationSubscription: any;
 
   /**
@@ -267,7 +266,7 @@ export abstract class AbstractControl {
   /**
    * The parent control.
    */
-  get parent(): FormGroup|FormArray {
+  get parent(): FormGroup|FormArray|null {
     return this._parent;
   }
 
@@ -1018,7 +1017,7 @@ export abstract class AbstractControl {
    */
   private _parentMarkedDirty(onlySelf?: boolean): boolean {
     const parentDirty = this._parent && this._parent.dirty;
-    return !onlySelf && parentDirty && !this._parent._anyControlsDirty();
+    return !onlySelf && !!parentDirty && !this._parent!._anyControlsDirty();
   }
 }