Prevent a fatal error if Router->current is empty

Prevents a fatal error ("Call to a member function getAction() on a non-object") if currentRouteAction() is called and there is no current route.  This adds code that is similar to a check found in `currentRouteName()`.  I've run into this error when other packages try to use `currentRouteAction()` and the current request is being handled by a `before()` filter.
Just `return;` is fine here instead of `return null;`