Don't transfer focus to NULL node on touch

Fixes #5185

sway/input/cursor.c

@@ -408,8 +408,10 @@ static void handle_touch_down(struct wl_listener *listener, void *data) {
 				event->touch_id, sx, sy);
 	}
 
+	if (focused_node) {
 		seat_set_focus(seat, focused_node);
 	}
+}
 
 static void handle_touch_up(struct wl_listener *listener, void *data) {
 	struct sway_cursor *cursor = wl_container_of(listener, cursor, touch_up);