fix(forms): make Validators.email support optional controls (#20869)

Bring email validator in line with other validators so that empty values are ignored.

PR Close #20869

packages/forms/src/validators.ts

@@ -125,6 +125,9 @@ export class Validators {
    * Validator that performs email validation.
    */
   static email(control: AbstractControl): ValidationErrors|null {
+    if (isEmptyInputValue(control.value)) {
+      return null;  // don't validate empty values to allow optional controls
+    }
     return EMAIL_REGEXP.test(control.value) ? null : {'email': true};
   }