seat: Handle unfocusing layer-surface with no other focusable view
roots_seat_set_focus would exit too early in case of view == NULL and prev_focus == NULL, not unfocusing a layer-surface that could, for instance, lose its keyboard-interactive status.
On top of that, roots_seat_set_focus_layer wouldn't even try to reset the focus in such cases.
Fixes phosh#161 (closed)