Add callIfNull support

index.js

@@ -14,7 +14,7 @@ module.exports = function (alternatives) {
 	} else if (arguments.length > 1) {
 		throw new Error(`Only one argument is accepted; maybe you forgot to wrap the different alternatives into an array?`);
 	} else {
-		return combinator((value, applyValidators) => {
+		let validator = combinator((value, applyValidators) => {
 			let allErrors = [];
 	
 			for (let alternative of alternatives) {
@@ -35,5 +35,9 @@ module.exports = function (alternatives) {
 
 			throw new ValidationError(`Must satisfy at least one of: ${errorList}`, { errors: allErrors });
 		});
+
+		validator.callIfNull = true;
+
+		return validator;
 	}
 };