TiDB gives hard-to-understand error message for definition with NULL in range partition

[YangKeao]

Bug Report

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1. Minimal reproduce step (Required)

mysql> create table a (col1 int, col2 int, unique key (col1, col2)) partition by range  columns (col1, col2) (partition p0 values less than (NULL, 1 ));
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 28 near ",1))"

Expected:

mysql> create table a (col1 int, col2 int, unique key (col1, col2)) partition by range  columns (col1, col2) (partition p0 values less than (NULL, 1));
ERROR 1566 (HY000): Not allowed to use NULL value in VALUES LESS THAN