code: function maybeModifier(predicate) {
          return function () {
            if (!check.defined(arguments[0]) || check.nulled(arguments[0])) {
              return true;
            }
            return predicate.apply(null, arguments);
          };
        }
docs: Public modifier `maybe`.
  
  Returns `true` if `predicate` is  `null` or `undefined`,
  otherwise propagates the return value from `predicate`.
  copied from check-types.js