[Bug] MySQL does not support the NULLS FIRST / NULLS LAST options

https://github.com/longbridgeapp/nested-set/blob/main/nested_set.go#L295

```
Error 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULLS FIRST, lft ASC FOR UPDATE'
```

https://learnsql.com/blog/how-to-order-rows-with-nulls/