Allow missing routes (App::missing())

Change it so if the route returned by the $router->current function is null, we just return empty vars from the currentRoute function. In my testing this works fine when we are using it for a 404 page handled by laravels App::missing functionality, and the unit tests all pass.

If there is anything you'd like to change let me know.

This is for the issue I created here https://github.com/davejamesmiller/laravel-breadcrumbs/issues/40