error occurred when the different row's json column is NULL in the one db source, but is not NULL in the other db source

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

upstream:

create database if not exists json_test;
create table json_test.test (a int, b json, primary key(a));

insert into json_test.test values (1, '{"id": 1, "name":"aaa"}');

downstream:

create database if not exists json_test;
create table json_test.test (a int, b json, primary key(a));

insert into json_test.test values (1, NULL);

3. What did you expect to see?
   check different

4. What did you see instead?
   error occurred:

Error in comparison process:
unmarshal json  failed, error unexpected end of JSON input error occured in `test`.`t`

5. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   master
6. which tool are you using?
   sync-diff-inspector
7. what versionof tool are you using (pump -V or tidb-lightning -V or syncer -V)?