The conv function behaves inconsistently with both tidb and tikv in some cases

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `t1` (
  `id` char(10) NOT NULL,
  PRIMARY KEY (`id`) /*T![clustered_index] NONCLUSTERED */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin 
insert into t1 values ("111");
set @@session.tidb_isolation_read_engines = "tiflash";
select * from t1 where conv(id,90,10) ="111";

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

mysql> select * from t1 where conv(id,90,10) ="111";
Empty set (0.00 sec)

mysql> select * from t1 where conv(id,90,10) is null;
+-----+
| id  |
+-----+
| 111 |
+-----+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> select * from t1 where conv(id,90,10) ="111";
+-----+
| id  |
+-----+
| 111 |
+-----+
1 row in set (0.01 sec)

mysql> select * from t1 where conv(id,90,10) is null;
Empty set (0.01 sec)

4. What is your TiFlash version? (Required)

6.1.0

Start by running the supplied CREATE TABLE, INSERT, isolation-setting, and CONV queries against TiFlash 6.1.0 and compare the results with the expected MySQL behavior. Trace the CONV handling in the relevant TiFlash expression path; done means the equality and IS NULL queries produce the expected results, with regression coverage for this reproducer.