[nerv_]

Shitbox2, тогда еще проще

function modifier(any) {
	return any === undefined ? null : any;
}

function isValid(any) {
	return modifier(any);
}

alert(isValid(0));
alert(isValid(1));
alert(isValid(undefined));

Я бы так сделал

function isUndefined(any) {
	return any === undefined;
}

function Validator({message}) {
	this.message = isUndefined(message) ? null : message;
}

let validator1 = new Validator({message:'test'});
let validator2 = new Validator({message: undefined});
let validator3 = new Validator({});

alert(validator1.message);
alert(validator2.message);
alert(validator3.message);