[Bug] MySQL 后端导入自带的测试 NULL 数据报转换错误

### Bug Type (问题类型)

exception / error (异常报错)

### Before submit

- [X] I had searched in the [issues](https://github.com/apache/hugegraph-toolchain/issues) and found no similar issues.

### Environment (环境信息)

- Server Version: v1.x.x
- Toolchain Version: v1.x.x
- Data Size: 6 vertices, 2 edges

### Expected & Actual behavior (期望与实际表现)

使用的是hugegraph-loader 下面static/example/mysql 的配置导入的
第6个person tom 的数据 age通过mysql reader读进去是 NULL_STR = "NULL"

会触发异常
java.lang.IllegalArgumentException: Failed to convert value(key=age) 'NULL'(class java.lang.String) to Number

### Vertex/Edge example (问题点 / 边数据举例)

```javascript
(1,'marko',29,'Beijing'),(2,'vadas',27,'HongKong'),(3,'josh',32,'Beijing'),(4,'peter',35,'Shanghai'),(5,'li,nary',26,'Wu,han'),(6,'tom',NULL,NULL)
```

### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

```javascript
{
"vertices": [
{
"label": "person",
"input": {
"type": "jdbc",
"vendor": "mysql",
"driver": "com.mysql.cj.jdbc.Driver",
"url": "jdbc:mysql://127.0.0.1:3306",
"database": "example",
"table": "person",
"username": "root",
"password": "",
"batch_size": 500
},
"id": "id",
"null_values": ["NULL"]
},
{
"label": "software",
"input": {
"type": "jdbc",
"vendor": "mysql",
"driver": "com.mysql.cj.jdbc.Driver",
"url": "jdbc:mysql://127.0.0.1:3306",
"database": "example",
"table": "software",
"username": "root",
"password": "",
"batch_size": 500
},
"id": "id"
}
],
"edges": [
{
"label": "knows",
"source": ["source_id"],
"target": ["target_id"],
"input": {
"type": "jdbc",
"vendor": "mysql",
"driver": "com.mysql.cj.jdbc.Driver",
"url": "jdbc:mysql://127.0.0.1:3306",
"database": "example",
"table": "knows",
"username": "root",
"password": "",
"batch_size": 500
},
"ignored": ["id"]
},
{
"label": "created",
"source": ["source_id"],
"target": ["target_id"],
"input": {
"type": "jdbc",
"vendor": "mysql",
"driver": "com.mysql.cj.jdbc.Driver",
"url": "jdbc:mysql://127.0.0.1:3306",
"database": "example",
"table": "created",
"username": "root",
"password": "",
"batch_size": 500
},
"ignored": ["id"]
}
]
}
```