To suppress the footer I have 2 ideas.
1. Use the select expert to not select those records
Not IsNull({YourField})
2. Use a formula
If IsNull({YourField}) then
0
Else
1
Suppress the gorup footer with
Sum({@NewFormula},{GroupField}) = 0
The Test above in the section expert needs to be
IsNull({YourField})
mlmcc