validator method
A function that must return a validation error if the provided
value is invalid and null otherwise.
Implementation
@override
ConfirmPasswordError? validator(String value) {
final trimmedValue = value.trim();
final trimmedOriginal = originalPassword.trim();
if (trimmedValue.isEmpty) return ConfirmPasswordError.empty;
if (trimmedValue != trimmedOriginal) return ConfirmPasswordError.mismatch;
return null;
}