docs(material/form-field): custom MatFormFieldControl validation not working unless touched (#26873)

Fixes a bug in the Angular Material  where form-control remains valid unless touched. This is because MatFormFieldControl emits null value when form is invalid, but default value was not set to null, bypassing required Validators.

Fixes #23352

Author: Captainnik53

src/components-examples/material/form-field/form-field-custom-control/form-field-custom-control-example.ts

@@ -46,7 +46,7 @@ import {MatIconModule} from '@angular/material/icon';
 })
 export class FormFieldCustomControlExample {
   form: FormGroup = new FormGroup({
-    tel: new FormControl(new MyTel('', '', '')),
+    tel: new FormControl(null),
   });
 }