Function INSERT should be NULL if any argument is NULL

[SunRunAway]

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.

SELECT INSERT(0.0024912753641791164, 0.00000000000000000000, 1, NULL);

2. What did you expect to see?

Due to https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_insert,

Returns NULL if any argument is NULL.

mysql> SELECT INSERT(0.0024912753641791164, 0.00000000000000000000, 1, NULL);
+----------------------------------------------------------------+
| INSERT(0.0024912753641791164, 0.00000000000000000000, 1, NULL) |
+----------------------------------------------------------------+
| NULL                                                           |
+----------------------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> SELECT INSERT(0.0024912753641791164, 0.00000000000000000000, 1, NULL);
+----------------------------------------------------------------+
| INSERT(0.0024912753641791164, 0.00000000000000000000, 1, NULL) |
+----------------------------------------------------------------+
| 0.0024912753641791164                                          |
+----------------------------------------------------------------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   fdbc149