what comes to mind is create a formula and use that for the grouping, instead of the field in the database.
you could create a formula like:
if isnull({table.field}) then
cdate(1900, 1, 1) //or whatever date you like
else
{table.field}
this will group all of your null values together and you can create a suppression formula to not display the date in the report for the nulls...but they will group.
HTH