MDEV-40698 Fix ROLLUP query results with empty result set (testfix)

Like b3830bce9a, this corrects the ROLLUP
result of the duckdb.duckdb_sql_syntax test to have a NULL row when
there is an empty result set.

storage/duckdb/mysql-test/duckdb/r/duckdb_sql_syntax.result

@@ -1,6 +1,7 @@
 CREATE TABLE t1 (id INT PRIMARY KEY, col1 INT, col2 INT);
 SELECT id, col1, col2 FROM t1 GROUP BY id, col1, col2 WITH ROLLUP;
 id	col1	col2
+NULL	NULL	NULL
 ALTER TABLE t1 ENGINE=DuckDB;
 SELECT id, col1, col2 FROM t1 GROUP BY id, col1, col2 WITH ROLLUP;
 id	col1	col2