Window function panicked when partition by NULL

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

CREATE TABLE t1 (a int, b float);

INSERT INTO t1 VALUES
   (4,12),(5,2),(9,14),(12,10),(6,8),(5,7),(2,15),(2,15),(15,11),(14,5),(14,240),
   (1,10),(14,5),(5,9),(1,11),(2,5),(11,9),(13,13),(6,187),(8,12),(10,12),(15,14),
   (50,4),(14,3),(166,2),(15,13),(10,12),(48,4),(13,3),(7,10);

SELECT a, b, LEAD (b, 1) OVER (ORDER BY a,b ASC) AS c FROM t1 a ORDER BY a, b, c;
SELECT a, b, LEAD (b, 1) OVER (PARTITION BY NULL ORDER BY a,b ASC) AS c FROM t1 a ORDER BY a, b, c;

2. What did you expect to see?

The two select return same result.

3. What did you see instead?

The second one panicked.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

master 373317f