Goldengate Syntax for NULL check on user FILTER (ON UPDATE Question

@NULL

 Can anyone provide assistance on syntax associate with filter records on DML update for where a field on the record is null

I know you can use @NULL with where feature but not sure how to combine that with FILTER and ON UPDATE

Following does not work

table rcaadmin.rca_file_tj, filter (ON UPDATE, c_card_serial_number = @NULL);

I am looking to replicate updates only where c_card_serial_number is null.

Thanks

Terry