fix(ngAnimate): TypeError Cannot call method 'querySelectorAll' in cancelChildAnimations

When an element containing both ng-repeat and ng-if directives attempts to remove any items from
the repeat collection, the following error is thrown: "TypeError Cannot call method 'querySelectorAll'
of undefined".  This happens because the cancelChildAnimations code naively belives that the jqLite
object always has an element node within it. The fix in this commit addresses to securely check to see
if a node was properly extracted before any child elements are inspected.

Closes #6205

src/ngAnimate/animate.js

@@ -942,16 +942,21 @@ angular.module('ngAnimate', ['ng'])
 
       function cancelChildAnimations(element) {
         var node = extractElementNode(element);
-        forEach(node.querySelectorAll('.' + NG_ANIMATE_CLASS_NAME), function(element) {
-          element = angular.element(element);
-          var data = element.data(NG_ANIMATE_STATE);
-          if(data && data.active) {
-            angular.forEach(data.active, function(operation) {
-              (operation.done || noop)(true);
-              cancelAnimations(operation.animations);
-            });
-          }
-        });
+        if (node) {
+          var nodes = angular.isFunction(node.getElementsByClassName) ?
+            node.getElementsByClassName(NG_ANIMATE_CLASS_NAME) :
+            node.querySelectorAll('.' + NG_ANIMATE_CLASS_NAME);
+          forEach(nodes, function(element) {
+            element = angular.element(element);
+            var data = element.data(NG_ANIMATE_STATE);
+            if(data && data.active) {
+              angular.forEach(data.active, function(operation) {
+                (operation.done || noop)(true);
+                cancelAnimations(operation.animations);
+              });
+            }
+          });
+        }
       }
 
       function cancelAnimations(animations) {

test/ngAnimate/animateSpec.js

@@ -3370,5 +3370,21 @@ describe("ngAnimate", function() {
         expect(stat).toBe('gone');
       });
     });
+
+    it('should not throw an error when only comment nodes are rendered in the animation',
+      inject(function($rootScope, $compile) {
+
+      $rootScope.items = [1,2,3,4,5];
+
+      var element = html($compile('<div><div class="animated" ng-if="valid" ng-repeat="item in items"></div></div>')($rootScope));
+
+      $rootScope.$digest();
+
+      $rootScope.items = [];
+
+      $rootScope.$digest();
+
+      expect(element.children().length).toBe(0);
+    }));
   });
 });