Search for empty fields

IMHO currently it is not possible to search for records with empty fields
Example - try to find unasigned cases.
creating a menu with domain ('state','=','open'),('user_id','=',False)] returns the unasigned cases, but it is not possible to select/assign users, because the many2one list is empty (due to the userid=False)
It seems to be a catch
allowing to select a responsible user will make disappear the case from the form with
unasigned cases, which IMHO is problematic.
IMHO the modified records should be visible until the next (manual) refresh.
see also discussion https://bugs.launchpad.net/bugs/367471