sync json null failed

[giant-panda666]

What did you do?

1. create table in MySQL

CREATE TABLE `t0` (
  `id` bigint(20) unsigned NOT NULL COMMENT '主键',
  `position` json NOT NULL COMMENT '标记位置',
   PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

use t0 to replace real table here, but bug is still the same.
2. create dm sync task sync t0 to TiDB with mode all
3. insert data into t0

insert into t0 values(1, cast('null' as json));

What did you expect to see?

data inserted in MySQL is synced to TiDB

What did you see instead?

i got error: Error 3140 (22032): Invalid JSON text: The document is empty

Versions of the cluster

DM version (run dmctl -V or dm-worker -V or dm-master -V):

v8.5.0

Upstream MySQL/MariaDB server version:

MySQL 5.7

Downstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

v7.5.1

How did you deploy DM: tiup or manually?

TiDB Operator

Other interesting information (system version, hardware config, etc):

>
>

current status of DM cluster (execute query-status <task-name> in dmctl)

"errors": [
                            {
                                "ErrCode": 10006,
                                "ErrClass": "database",
                                "ErrScope": "not-set",
                                "ErrLevel": "high",
                                "Message": "startLocation: [position: (mysql-bin|000001.000787, 905342), gtid-set: 64db65da-36f3-11f0-978b-9c63c0933e6c:1-3699301], endLocation: [position: (mysql-bin|000001.000787, 905788), gtid-set: 64db65da-36f3-11f0-978b-9c63c0933e6c:1-3699301]: execute statement failed: REPLACE INTO `cactus`.`marks` (`id`,`user_id`,`spu_id`,`spu_type_id`,`meta`,`post_id`,`created_at`,`updated_at`,`deleted_at`,`hobby_id`,`extra`,`mark_at`,`related_id`,`mark_type`,`position`,`post_attachment_id`) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)",
                                "RawCause": "Error 3140 (22032): Invalid JSON text: The document is empty",
                                "Workaround": ""
                            }
                        ],