st/focus-manager: Check for NULL instead of stage to exit loop

In st_focus_manager_stage_event() we get the current key-focus actor from the stage, and then enter a loop where we iterate through parents until we get to the stage. If the key-focus actor is not a descendent of the stage for some reason, this loop will never exit and we've locked up.

We can easily prevent the endless loop by checking for NULL as the exit condition instead of checking for the stage, so do that instead.