insert null into not-null generated column leads to wrong index record

[joechenrh]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table t3;
create table t3(
    id int primary key,
    c1 varchar(16) default null,
    c2 varchar(16) GENERATED ALWAYS AS (concat(c1, c1)) VIRTUAL NOT NULL,
    KEY idx (c2)
);

insert into t3(id, c1) values(1, "aaaa");
insert ignore into t3 set id = 1, c1 = "bbbb" on duplicate key update id = 2, c1 = null;
insert into t3 set id = 2, c1 = "cccc" on duplicate key update c1 = "dddd";

2. What did you expect to see? (Required)

No error.

3. What did you see instead (Required)

ERROR 8141 (HY000): assertion failed: key: 7480000000000000715f69800000000000000100038000000000000002, assertion: Exist, start_ts: 454804733357457409, existing start ts: 0, existing commit ts: 0

4. What is your TiDB version? (Required)