pg_partition_tree crashes for a non-defined relation

Hi all,

While testing another patch, I have bumped into the issue of
$subject... I should have put some more negative testing from the start
on this stuff, here is a culprit query when passing directly an OID:
select pg_partition_tree(0);

I think that we should make the function return NULL if the relation
defined does not exist, as we usually do for system-facing functions.
It is also easier for the caller to know that the relation does not
exist instead of having a plpgsql try/catch wrapper or such.

Thoughts?
--
Michael