fix(traversing): parent() may fail in functions (#1637)

Author: 5saviahv

lib/api/traversing.js

@@ -86,7 +86,7 @@ exports.parent = function (selector) {
     }
   });
 
-  if (arguments.length) {
+  if (selector) {
     set = exports.filter.call(set, selector, this);
   }
 

test/api/traversing.js

@@ -619,6 +619,12 @@ describe('$(...)', function () {
       expect(result[1].attribs.id).toBe('vegetables');
     });
 
+    it('(undefined) : should not throw an exception', function () {
+      expect(function () {
+        $('li').parent(undefined);
+      }).not.toThrow();
+    });
+
     it('() : should return an empty object for top-level elements', function () {
       var result = $('html').parent();
       expect(result).toHaveLength(0);