Uncorrect result when there's union

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a timestamp, b datetime);
insert into t values('2020-07-29 09:07:01', '2020-07-27 16:57:36');
select greatest(a, b) from t union select null;

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

+---------------------+
| greatest(a, b)      |
+---------------------+
| NULL                |
| 2020-07-29 09:07:01 |
+---------------------+

3. What did you see instead (Required)

+----------------+
| greatest(a, b) |
+----------------+
| NULL           |
|                |
+----------------+
2 rows in set, 1 warning (0.01 sec)

mysql> show warnings;
+---------+------+-----------------------------------------+
| Level   | Code | Message                                 |
+---------+------+-----------------------------------------+
| Warning | 1406 | Data Too Long, field len 0, data len 19 |
+---------+------+-----------------------------------------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

current master

[ti-srebot]

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