required<T> static method
Implementation
static FormFieldValidator<T> required<T>({String? message}) => (T? value) {
if (value == null) return message;
if (value is String && value.isEmpty) return message ?? U.s.required;
if (value is List && value.isEmpty) return message ?? U.s.required;
if (value is Map && value.isEmpty) return message ?? U.s.required;
return null;
};