2 thoughts come to mind:
1) File/Options/Reporting check the convert null values to default
2) create a formula with local variables
local numbervar gdr := {Employees.GroupDisabilityRate};
//check if the variable is null, if so assign the 0
if isnull(gdr) then gdr := 0;
//finally, add them all up gdr+...
Hope one of these ways is useful.