PG17 Compatibility - Fix HideCitusDependentObjects function (#7796)

There is a crash when running vanilla tests because of the
`citus.hide_citus_dependent_objects` GUC. We turn on this GUC only for
the pg vanilla tests. This GUC runs the following function
`HideCitusDependentObjectsOnQueriesOfPgMetaTables`. This function
doesn't take into account the new `mergeJoinCondition`. I rewrote the
function such that it checks for merge join conditions as well.

Relevant PG commit:
https://github.com/postgres/postgres/commit/0294df2f1

The crash could be reproduced locally like the following:
```SQL
SET citus.hide_citus_dependent_objects TO on;

CREATE OR REPLACE FUNCTION
    pg_catalog.is_citus_depended_object(oid,oid)
    RETURNS bool
    LANGUAGE C
    AS 'citus', $$is_citus_depended_object$$;

-- try a system catalog
MERGE INTO pg_class c
USING (SELECT 'pg_depend'::regclass AS oid) AS j
ON j.oid = c.oid
WHEN MATCHED THEN
UPDATE SET reltuples = reltuples + 1
RETURNING j.oid;

CREATE VIEW classv AS SELECT * FROM pg_class;

MERGE INTO classv c
USING pg_namespace n
ON n.oid = c.relnamespace
WHEN MATCHED AND c.oid = 'pg_depend'::regclass THEN
UPDATE SET reltuples = reltuples - 1
RETURNING c.oid;
-- crash happens here
```

src/backend/distributed/utils/citus_depended_object.c

@@ -243,12 +243,24 @@ HideCitusDependentObjectsOnQueriesOfPgMetaTables(Node *node, void *context)
 
 				if (OidIsValid(metaTableOid))
 				{
+					bool mergeJoinCondition = false;
+#if PG_VERSION_NUM >= PG_VERSION_17
+
+					/*
+					 * In Postgres 17, the query tree has a specific field for the merge condition.
+					 * So we shouldn't modify the jointree, but rather the mergeJoinCondition here
+					 * Relevant PG17 commit: 0294df2f1
+					 */
+					mergeJoinCondition = query->mergeJoinCondition;
+#endif
+
 					/*
 					 * We found a valid pg meta class in query,
 					 * so we assert below conditions.
 					 */
-					Assert(query->jointree != NULL);
-					Assert(query->jointree->fromlist != NULL);
+					Assert(mergeJoinCondition ||
+						   (query->jointree != NULL &&
+							query->jointree->fromlist != NULL));
 
 					Node *citusDependentObjExpr =
 						CreateCitusDependentObjectExpr(varno, metaTableOid);
@@ -257,9 +269,19 @@ HideCitusDependentObjectsOnQueriesOfPgMetaTables(Node *node, void *context)
 					 * We do not use security quals because a postgres vanilla test fails
 					 * with a change of order for its result.
 					 */
+					if (!mergeJoinCondition)
+					{
 						query->jointree->quals = make_and_qual(
 							query->jointree->quals, citusDependentObjExpr);
 					}
+					else
+					{
+#if PG_VERSION_NUM >= PG_VERSION_17
+						query->mergeJoinCondition = make_and_qual(
+							query->mergeJoinCondition, citusDependentObjExpr);
+#endif
+					}
+				}
 
 				MemoryContextSwitchTo(originalContext);
 			}