[Bug Fix] SEGV on query with Left Outer Join (#7787) (#7901)

DESCRIPTION: Fixes a crash in left outer joins that can happen when
there is an an aggregate on a column from the inner side of the join.

Fix the SEGV seen in #7787 and #7899; it occurs because a column in the
targetlist of a worker subquery can contain a non-empty varnullingrels
field if the column is from the inner side of a left outer join. The
issue can also occur with the columns in the HAVING clause, and this is
also tested in the fix. The issue was triggered by the introduction of
the varnullingrels to Vars in Postgres 16 (2489d76c)

There is a related issue, #7705, where a non-empty varnullingrels was
incorrectly copied into the query tree for the combine query. Here, a
non-empty varnullingrels field of a var is incorrectly copied into the
query tree for a worker subquery.

The regress file from #7705 is used (and renamed) to also test this
(#7787). An alternative test output file is required for Postgres 15
because of an optimization to DISTINCT in Postgres 16 (1349d2790bf).

src/backend/distributed/planner/query_pushdown_planning.c

@@ -2097,6 +2097,16 @@ CreateSubqueryTargetListAndAdjustVars(List *columnList)
 		 */
 		column->varno = 1;
 		column->varattno = resNo;
+
+		/*
+		 * 1 subquery means there is one range table entry so with Postgres 16+ we need
+		 * to ensure that column's varnullingrels - the set of join rels that can null
+		 * the var - is empty. Otherwise, when given the query, the Postgres planner
+		 * may attempt to access a non-existent range table and segfault, as in #7787.
+		 */
+#if PG_VERSION_NUM >= PG_VERSION_16
+		column->varnullingrels = NULL;
+#endif
 	}
 
 	return subqueryTargetEntryList;