[BUG] toDriver method never been called on customType

### What version of `drizzle-orm` are you using?

0.30.8

### What version of `drizzle-kit` are you using?

0.20.10

### Describe the Bug

[Context] I have an application where I will change the database type. [mongo -> postgresql] So until that point, I'm using [Mongo FWD](https://github.com/EnterpriseDB/mongo_fdw) to keep data synced between databases and multiple services.

So I have to use the `customType` function from drizzle-orm/pg-core and I defined the `toDriver` method to convert values. I have to cover cases when the incoming value is null, and I have to convert it here because of the type differences.

And I realized even if I define a method to convert data, when I'm inserting, the toDriver is never called if the value is `null`.

https://github.com/drizzle-team/drizzle-orm/blob/3bbf9ed5f4e5ec6b1fa6f303714ad02bd98f6b6e/drizzle-orm/src/sql/sql.ts#L206C1-L207C1

For me, it seems logical that if the `toDriver` is defined, then it should be called whatever the value is because it's created on purpose.

### Expected behavior

If the toDriver method is defined on a customType, then it has to be called with any type of value.

### Environment & setup

_No response_