Fix join columnar with heap for PG16

[ivan-v-kush]

Hydra is broken when tries join columnar with heap tables. This PR fixes it.

CREATE TABLE tbl1 (a integer);
CREATE TABLE tbl2 (a integer);
CREATE TABLE tbl3 (b integer) using columnar;
SELECT *
FROM tbl1
	LEFT JOIN tbl2 ON tbl1.a = tbl2.a
	LEFT JOIN tbl3 ON tbl2.a = tbl3.b;

the server suddenly closed the connection
        Most likely, the server stopped working due to a glitch.
        before or during the execution of the request.
Connection to the server is lost. The recovery attempt failed.
Connection to the server is lost. The recovery attempt failed.
!?> \q

This error also exists in Citus
citusdata/citus#7647

outer_join_rels were introduced in postgres/postgres@2489d76