tidb 7.5 无法转换string，行为与mysql不一致

5.7/8.0测试结果同上，

mysql> select cast(compress('b') as char);
+-----------------------------+
| cast(compress('b') as char) |
+-----------------------------+
|    x▒K  c c               |
+-----------------------------+
1 row in set (0.02 sec)

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.32    |
+-----------+
1 row in set (0.00 sec)


mysql> select cast(compress('b') as char);
+-----------------------------+
| cast(compress('b') as char) |
+-----------------------------+
|    x▒K  c c                |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select version();
+------------+
| version()  |
+------------+
| 5.7.31-log |
+------------+
1 row in set (0.00 sec)