Fix crash on create statistics when it's not created on relation

This crash has been there for a while, but wasn't tested before pg18.
PG18 added this test:

CREATE STATISTICS tst ON a FROM (VALUES (x)) AS foo;

which tries to create statistics on a derived-on-the-fly table
(which is not allowed)

However Citus assumes we always have a valid table when intercepting
CREATE STATISTICS command to check for Citus tables

Added a check to return early if needed

src/backend/distributed/commands/statistics.c

@@ -70,6 +70,12 @@ PreprocessCreateStatisticsStmt(Node *node, const char *queryString,
 	CreateStatsStmt *stmt = castNode(CreateStatsStmt, node);
 
 	RangeVar *relation = (RangeVar *) linitial(stmt->relations);
+
+	if (!IsA(relation, RangeVar))
+	{
+		return NIL;
+	}
+
 	Oid relationId = RangeVarGetRelid(relation, ShareUpdateExclusiveLock, false);
 
 	if (!IsCitusTable(relationId) || !ShouldPropagate())