zink: fix descriptor update flagging on null ssbo set

this could unset the update flag if the last ssbo was null, which would
be a bug if it could ever be triggered

found by inspection

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21739>
(cherry picked from commit 7145ccfa0596a2c4adb707f657161fe3ba5e09af)