fix(ngAnimate): Change to falsy value

- Change addClass & removeClass to pass in empty string to setClass so
  $$setClassImmediately does not trigger due to arrays always being
truthy

- Change to use `null` instead

Author: wesleycho

src/ngAnimate/animate.js

@@ -1077,7 +1077,7 @@ angular.module('ngAnimate', ['ng'])
          * @return {Promise} the animation callback promise
         */
         addClass: function(element, className, options) {
-          return this.setClass(element, className, [], options);
+          return this.setClass(element, className, null, options);
         },
 
         /**
@@ -1111,7 +1111,7 @@ angular.module('ngAnimate', ['ng'])
          * @return {Promise} the animation callback promise
         */
         removeClass: function(element, className, options) {
-          return this.setClass(element, [], className, options);
+          return this.setClass(element, null, className, options);
         },
 
         /**