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

When element with ng-repeat has an ng-if directive and user try to remove any item from array used for ng-repeat, he gets error "TypeError Cannot call method 'querySelectorAll' of undefined". This happens because in method cancelChildAnimations of ngAnimate directive not checked value returned from extractElementNode(element) method.
Fix add a validation for result of extractElementNode(element) method.

Closes #6205