`WEEK` should return week even when the `mode` is `NULL`

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

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

mysql> SELECT WEEK('2023-01-01', NULL);
+--------------------------+
| WEEK('2023-01-01', NULL) |
+--------------------------+
|                        1 |
+--------------------------+
1 row in set (0.06 sec)

3. What did you see instead (Required)

mysql> SELECT WEEK('2023-01-01', NULL);
+--------------------------+
| WEEK('2023-01-01', NULL) |
+--------------------------+
|                     NULL |
+--------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[zanmato1984]

/severity moderate