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?