Ensure pg_filenode_relation(0, 0) returns NULL.

Ensure pg_filenode_relation(0, 0) returns NULL.

Previously, a zero value for the relfilenode resulted in
a confusing error message about "unexpected duplicate".
This function returns NULL for other invalid relfilenode
values, so zero should be treated likewise.

It's been like this all along, so back-patch to all supported
branches.

Justin Pryzby

Discussion: https://postgr.es/m/20210612023324.GT16435@telsasoft.com