For example, with the following formula:

if {CnLGf_1.CnLGf_1_Amount} > 0 then "Donors" else "Non-donors"

If the {CnLGf_1.CnLGf_1_Amount} field is NULL, then the formula will return a blank value.

Solution: Check for null values at the beginning of the formula

if NOT IsNULL({CnLGf_1.CnLGf_1_Amount}) 
then ( if {CnLGf_1.CnLGf_1_Amount}  > 0 then "Donors")
else "Non-donors"​