kiosk-shell: Don't attempt to activate a window

Without a active_surface_tree there's no point in attempting to access
the shell surfaces list, nor we're going to to have a temorary list
based on it.

This might happen in situations where
kiosk_shell_output_set_active_surface_tree() is being passed a NULL shell
surface root.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>