goctl model mysql datasource fails to identify NULL correctly in MariaDB

[leetpy]

Describe the bug
When using goctl model mysql datasource to generate database models, the tool determines if a field's default value is NULL by checking COLUMN_DEFAULT. However, this logic encounters issues when running against MariaDB.

To Reproduce
Steps to reproduce the behavior, if applicable:

1. test sql

CREATE DATABASE testnull;
USE testnull;

CREATE TABLE `test_table` (
 `test_null` VARCHAR(10) DEFAULT 'NULL',
 `name` VARCHAR(10) DEFAULT 'aaa',
 `age` int DEFAULT 10,
 `deleted_at` datetime(3) DEFAULT NULL
);

SELECT 
 c.COLUMN_NAME,
 c.DATA_TYPE,
 c.COLUMN_TYPE,
 c.EXTRA,
 c.COLUMN_COMMENT,
 c.COLUMN_DEFAULT,
 c.IS_NULLABLE,
 c.ORDINAL_POSITION 
FROM INFORMATION_SCHEMA.COLUMNS c 
WHERE c.TABLE_SCHEMA = 'testnull' 
AND c.TABLE_NAME = 'test_table';

2. The error is
   In mariadb-11.4 I get the flllowing content
   

In MySQL-9.6.0 I get the flllowing content

Expected behavior

The code check isDefaultNull with COLUMN_DEFAULT , but in mariadb, we get a null string.

https://github.com/zeromicro/go-zero/blob/master/tools/goctl/model/sql/parser/parser.go#L355

Environments (please complete the following information):

• OS: Linux
• goctl version 1.9.2