Filtering for null values

Hello,

I am trying to write a GraphQL filter to filter for nulls.

```
{
events(
filter: {
bookingId: { eq: null }
}
) {
id
booking {
id
}
}
}
```

But when I'm generating the filter expression, the SQL expression looks like `"WHERE bookingId = 'null'"` instead of `bookingId = null` without the single quotes.

Can this be fixed or can you point to where the toString is coming from?

Start by tracing how the GraphQL filter input for eq: null becomes a filter expression and then SQL, focusing on where the null value is converted to text. Done means the generated SQL no longer quotes the null value and the behavior is verified for this filter case.

索引モデルが issue の本文から書いたものです。