sequences not handled correctly when privacy by default is active

When trying to use pg_dump with privacy default is active, the code will treat sequences as not having a security label and null them. That results in an invalid dump that cannot be restored and prints out an error like this:

SELECT pg_catalog.setval('foo."bar_id_seq"', , false)

To workaround, I added:

 if attname == "last_value" {
     return (attname.to_string(), false);
 }

to masking.rs inside value_for_att but that is very hack-y. No idea how to fix this correctly.