pg_replication_slot_advance to return NULL instead of 0/0 if slot not advanced

Hi all,

When attempting to use multiple times pg_replication_slot_advance on a
slot, then the caller gets back directly InvalidXLogRecPtr as result,
for example:
=# select * from pg_replication_slot_advance('popo', 'FF/0');
 slot_name |  end_lsn
-----------+-----------
 popo      | 0/60021E0
(1 row)
=# select * from pg_replication_slot_advance('popo', 'FF/0');
 slot_name | end_lsn
-----------+---------
 popo      | 0/0
(1 row)

Wouldn't it be more simple to return NULL to mean that the slot could
not be moved forward?  That would be easier to parse for clients.
Please see the attached.

Thanks,
--
Michael