You can group the column data into null and not null and then perform percent of total on the fields in the table.
1. Create a calculated field to group the column data as below. (Here Order id is the column i want to check for nulls, similarly you can give any attribute/measure )
2. Then use the group to display only those with Not Null by excluding them.
3. Now perform quick table calculation to find percent of total.
Hope this helps you.