You may use the IsNull function to test the field and construct the If-Then-Else around your existing formula
Extracted from Crystal Reports help
Basic and Crystal syntax.
fld is any valid database, memo, or BLOB field.
Evaluates the field specified in the current record and returns TRUE if the field contains a null value.
You can use this function in a record selection formula to limit the report to records that have something other than a null value in the field specified. You can also use it to have the program take some action whenever it encounters a null value.
Some databases support null data values and others do not. This function will not work if the active database(s) do not support null values. If the database(s) do support null values, the function can be used to reference the null values that get created as the result of a failed lookup while joining.
Note: Zero and blank (empty or " ") are not null values.