citus_prepare_pg_upgrade fails on non-existent pg_dist_object entries

We observed an issue during citus_prepare_pg_upgrade where pg_dist_object references a non-existent objects:

```sql
UPDATE citus.pg_dist_object SET (type, object_names, object_args) = (SELECT * FROM pg_identify_object_as_address(classid, objid, objsubid))
ERROR: invalid role OID: 73072
```

This may have been due to a combination of CREATE/DROP ROLE and setting changes.

Related issue: we observed workers have an arbitrary set of pg_dist_object records.