In the Group Footer section, put something like this into the Control Source
of your text box:
=IIf(Count("*")=1, Null, Sum([Amount]))
If you have a label that reads "Total:" and you need to suppress that label
as well, right-click it to Change To | Text Box, and set its Control Source
to:
=IIf([txtTotal] Is Null, Null, "Total:")
If you need more than that, you can suppress the entire section by setting
its Visible property in its Format event based on the value of Count("*").