Unfortunately not that I know of.
Usually when you have a special condition around a multiselect list, such as also giving the option of searching on nulls, in your values you have to union all on a choice at the end
eg
SELECT Value, DisplayText
FROM MyList
UNION ALL
SELECT -1, '-No Value-'
Then search for that option in your stored procedure. Otherwise you have to look at user control issues such as leaving the field blank.
Best Wishes, The Redman; If something helps, please help show it by voting, if it solves, bonus!