$case->owner( $q->param('owner') );
.....
$case->update;

##</code><code>##

$case->owner(NULL);
.....
$case->update;

##</code><code>##

Handoff::Cases->set_sql('unowned', qq{SELECT * FROM cases 
                                      WHERE owner IS NULL 

##</code><code>##

Handoff::Cases->set_sql('unowned', qq{SELECT * FROM cases 
                                      WHERE owner IS NULL OR owner=''