Fix crash when unmapping last child of a tabbed workspace

* Create layout T[view view]
* Move the cursor into the title bar area
* Close both views

Sway would crash because container_at_tabbed would attempt to divide by
zero when there are no children.

The children check isn't needed for the stacked function because it
doesn't divide anything by the number of children.

Fixes #2636.

sway/tree/container.c

@@ -202,6 +202,9 @@ static struct sway_container *container_at_tabbed(struct sway_node *parent,
 	}
 	struct sway_seat *seat = input_manager_current_seat(input_manager);
 	list_t *children = node_get_children(parent);
+	if (!children->length) {
+		return NULL;
+	}
 
 	// Tab titles
 	int title_height = container_titlebar_height();