chore($AnimateRunner): examine the document more carefully

Some internal tests were failing because the `$document[0]` value
was null. This fix ensures that the if statement surrounding that
is more careful.

src/ng/animateRunner.js

@@ -81,7 +81,11 @@ var $$AnimateRunnerFactoryProvider = function() {
 
       this._doneCallbacks = [];
       this._tick = function(fn) {
-        if ($document[0].hidden) {
+        var doc = $document[0];
+
+        // the document may not be ready or attached
+        // to the module for some internal tests
+        if (doc && doc.hidden) {
           timeoutTick(fn);
         } else {
           rafTick(fn);