message: docs(aio): Add null type to form validation example
  Closes
  PR Close
diff: @@ -5,7 +5,7 @@ import { AbstractControl, NG_VALIDATORS, Validator, ValidatorFn, Validators } fr
  // #docregion custom-validator
  /** A hero's name can't match the given regular expression */
  export function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {
  - return (control: AbstractControl): {[key: string]: any} => {
  + return (control: AbstractControl): {[key: string]: any} | null => {
  const forbidden = nameRe.test(control.value);
  return forbidden ? {'forbiddenName': {value: control.value}} : null;
  };
  @@ -22,7 +22,7 @@ export function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {
  export class ForbiddenValidatorDirective implements Validator {
  @Input('appForbiddenName') forbiddenName: string;
  - validate(control: AbstractControl): {[key: string]: any} {
  + validate(control: AbstractControl): {[key: string]: any} | null {
  return this.forbiddenName ? forbiddenNameValidator(new RegExp(this.forbiddenName, 'i'))(control)
  : null;
  }
  