conversion from enum to JSON is not compatible with MySQL

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists tbl_5;
create table tbl_5 (col_83 enum('Alice','Bob','Charlie','David'));
insert into tbl_5 values ('David'); 
alter table tbl_5 modify column col_83 json ;

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

MySQL:

mysql> alter table tbl_5 modify column col_83 json ;
ERROR 3140 (22032): Invalid JSON text: "Invalid value." at position 0 in value for column '#sql-1_9b.col_83'.

3. What did you see instead (Required)

TiDB:

mysql> alter table tbl_5 modify column col_83 json ;
Query OK, 0 rows affected (0.52 sec)

4. What is your TiDB version? (Required)

master

commit 6d69d13d10138b8f1eab1e36a669a43c390c714e (HEAD, upstream/master)
Author: disksing <i@disksing.com>
Date:   Wed Jun 9 14:02:28 2021 +0800

    store/tikv: cleanup dependency (#25283)

[ymkzpx]

/assign

[xiongjiwei]

maybe similar to #9999