support eliminating group_concat when group-by-items contain at least one unique column

[XuHuaiyu]

Feature Request

Is your feature request related to a problem? Please describe:
Group_concat will not be eliminated after #9967 , and the reason is mentioned in that PR.

Describe the feature you'd like:
Considering system variable group_concat_max_len in CONCAT_WS if it's introduced when eliminating group_concat.
When all the arguments accepted by group_concat are promised to be not-null, we can rewrite it into concat_ws.

mysql> select group_concat(null);
+--------------------+
| group_concat(null) |
+--------------------+
| NULL               |
+--------------------+
1 row in set (0.00 sec)

mysql> select concat_ws(",",null);
+---------------------+
| concat_ws(",",null) |
+---------------------+
|                     |
+---------------------+
1 row in set (0.00 sec)

mysql> select group_concat("abc", null);
+---------------------------+
| group_concat("abc", null) |
+---------------------------+
| NULL                      |
+---------------------------+
1 row in set (0.00 sec)

mysql> select concat_ws(",","abc",null);
+---------------------------+
| concat_ws(",","abc",null) |
+---------------------------+
| abc                       |
+---------------------------+
1 row in set (0.00 sec)

Describe alternatives you've considered:
Add a maxLen attribute whose default value is -1 for concat_ws, and check it to decide whether or not truncate the final result. If it's -1, we would not truncate it.
Set maxLen as group_concat.maxLen when eliminating the group_concat.

Teachability, Documentation, Adoption, Migration Strategy: