Minor: Fix invalid query in test

Author: alamb

datafusion/sqllogictest/test_files/order.slt

@@ -986,17 +986,26 @@ statement ok
 create table t(a0 int, a int, b int, c int) as values (1, 2, 3, 4), (5, 6, 7, 8);
 
 # expect this query to run successfully, not error
+query IIII
+select * from (select c, a, NULL::int as a0, b from t order by a, c) t1
+union all
+select * from (select c, NULL::int as a, a0, b from t order by a0, c) t2
+order by c, a, a0, b
+limit 2;
+----
+4 2 NULL 3
+4 NULL 1 3
+
 query III
 select * from (select c, a, NULL::int as a0 from t order by a, c) t1
 union all
 select * from (select c, NULL::int as a, a0 from t order by a0, c) t2
-order by c, a, a0, b
+order by c, a, a0
 limit 2;
 ----
 4 2 NULL
 4 NULL 1
 
-
 # Casting from numeric to string types breaks the ordering
 statement ok
 CREATE EXTERNAL TABLE ordered_table (