fix($animateCss): don't unregister events when none have been added

Previously, when an animation was closed because no animation styles
where found, it would call .off() with an empty string as the argument.

For both jquery/jqlite this is the same as calling .off() without any
argument, which deregisters all event listeners on an element.

Closes #13514

src/ngAnimate/animateCss.js

@@ -750,7 +750,7 @@ var $AnimateCssProvider = ['$animateProvider', function($animateProvider) {
         }
 
         // Remove the transitionend / animationend listener(s)
-        if (events) {
+        if (events && events.length) {
           element.off(events.join(' '), onAnimationProgress);
         }
 

test/ngAnimate/animateCssSpec.js

@@ -1502,6 +1502,39 @@ describe("ngAnimate $animateCss", function() {
               expect(elementOffSpy.mostRecentCall.args[0]).toBe(event);
             });
         });
+
+        they("should not add or remove $prop event listeners when no animation styles are detected",
+          [TRANSITIONEND_EVENT, ANIMATIONEND_EVENT], function(event) {
+            inject(function($animateCss, $timeout) {
+
+              progress = event === TRANSITIONEND_EVENT ? transitionProgress : keyframeProgress;
+
+              // Make sure other event listeners are not affected
+              var otherEndSpy = jasmine.createSpy('otherEndSpy');
+              element.on(event, otherEndSpy);
+
+              expect(elementOnSpy).toHaveBeenCalledOnce();
+              elementOnSpy.reset();
+
+              var animator = $animateCss(element, {
+                event: 'enter',
+                structural: true
+              });
+
+              expect(animator.$$willAnimate).toBeFalsy();
+
+              // This will close the animation because no styles have been detected
+              var runner = animator.start();
+              triggerAnimationStartFrame();
+
+              expect(elementOnSpy).not.toHaveBeenCalled();
+              expect(elementOffSpy).not.toHaveBeenCalled();
+
+              progress(element, 10);
+              expect(otherEndSpy).toHaveBeenCalledOnce();
+            });
+        });
+
       });
     });