seat/dnd: support null drag icon surfaces
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 <1d38d4b87e1e0dc77582aff8604ad867921ea8e0@igalia.com>

1ace/sway,1ace/sway,1ace/sway