short circuit isEmpty for null/undefined

underscore.js

@@ -761,6 +761,7 @@
   // Is a given array, string, or object empty?
   // An "empty" object has no enumerable own-properties.
   _.isEmpty = function(obj) {
+    if (obj == null) return true;
     if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
     for (var key in obj) if (_.has(obj, key)) return false;
     return true;