Fix regression from r23575 -- wl_notificationtimestamp didn't get updated anymore...

Fix regression from r23575 -- wl_notificationtimestamp didn't get updated anymore on update notification, leading to duplicate mails being sent out.
Doing a WHERE on x=NULL doesn't work; need to do 'x IS NULL' here.
Perhaps we should make the DB functions smarter to handle that case to do what we'd expect it to do (as we do turning arrays into 'IN (...)') but this'll fix it for now.