BO Webi report shows () for null values,Need to replace it with Null text

Hi there

I have a webi Report in version BO 4.1,which gives below out put but it also generate (), I need to replace it with Null Text. Please suggest what can I do to make this change.

Below is the variable which has been applied to format it to %.

=[Column name]+"

("+FormatNumber(([column name]*100);"#.#%")+")" ForAll ([Presentation Type])

Thank