seat/dnd: support null drag icon surfaces

As per the Wayland spec [1]:

> The icon surface is an optional (can be NULL) surface that provides an
> icon to be moved around with the cursor.

However, as of now Sway "start_drag" signal handler does not starts the
DND session unless a non-NULL drag icons is provided. This patch fixes
it by skipping handling of the drag icon if it is null.

Fixes #5509

[1] https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_data_device

Signed-off-by: Nick Diego Yamane <nickdiego@igalia.com>

sway/input/seat.c

@@ -470,10 +470,7 @@ static void handle_start_drag(struct wl_listener *listener, void *data) {
 	wl_signal_add(&wlr_drag->events.destroy, &drag->destroy);
 
 	struct wlr_drag_icon *wlr_drag_icon = wlr_drag->icon;
-	if (wlr_drag_icon == NULL) {
+	if (wlr_drag_icon != NULL) {
-		return;
-	}
-
 		struct sway_drag_icon *icon = calloc(1, sizeof(struct sway_drag_icon));
 		if (icon == NULL) {
 			sway_log(SWAY_ERROR, "Allocation failed");
@@ -495,6 +492,7 @@ static void handle_start_drag(struct wl_listener *listener, void *data) {
 		wl_list_insert(&root->drag_icons, &icon->link);
 
 		drag_icon_update_position(icon);
+	}
 	seatop_begin_default(seat);
 }