when sync to mysql, data may not consitent with timestamp data type if explicit_defaults_for_timestamp var diffreent

in tidb will like this explicit_defaults_for_timestamp is on as default

mysql> create table tm(id int auto_increment, t_timestamp TIMESTAMP, primary key(id));
Query OK, 0 rows affected (0.24 sec)

mysql> show create table tm;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                        |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tm    | CREATE TABLE `tm` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `t_timestamp` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> insert into tm(t_timestamp) values(null);
Query OK, 1 row affected (0.13 sec)

mysql> select * from tm;
+----+-------------+
| id | t_timestamp |
+----+-------------+
|  1 | NULL        |
+----+-------------+
1 row in set (0.01 sec)

in mysql if explicit_defaults_for_timestamp is off will like this
for mysql Default Value is ON(>= 8.0.2) or OFF(<= 8.0.1)

mysql> create table tm(id int auto_increment, t_timestamp TIMESTAMP, primary key(id));
Query OK, 0 rows affected (0.03 sec)

mysql> show create table tm;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                    |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tm    | CREATE TABLE `tm` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `t_timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> insert into tm(t_timestamp) values(null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from tm;
+----+---------------------+
| id | t_timestamp         |
+----+---------------------+
|  1 | 2018-05-31 12:55:51 |
+----+---------------------+
1 row in set (0.01 sec)

about explicit_defaults_for_timestamp

Start by reproducing the two explicit_defaults_for_timestamp table definitions and insert results against TiDB and MySQL. Then trace the synchronization path for MySQL TIMESTAMP defaults and define done as consistent handling of the differing NULL, default, and ON UPDATE behavior; no source file or test is named in the issue.