JOIN quals, just like WHERE quals, even if they reference every one of the
join's relations.  Now that we can reorder outer and inner joins, it's
possible for such a qual to end up being assigned to an outer join plan node,
and we mustn't have it treated as a join qual rather than a filter qual for
the node.  (If it were, the join could produce null-extended rows that it
shouldn't.)  Per bug report from Pelle Johansson.