TiDB doesn't handle sort by NULL correctly

[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.

drop table if exists sort_null_bug;
create table sort_null_bug(a int);
insert into sort_null_bug values(1), (2), (3), (6), (4);
select * from (select *, null as b from sort_null_bug) t order by b;
select * from (select *, null as b from sort_null_bug) t order by b, a;

2. What did you expect to see?

The two SELECT statements executes successfully.

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

[winoros]

This is the root cause of #10456