text: Title: Add support for filtering by null values
  Body: I couldn't find anything in documentation how filtering fields with null works.
  
  I got it working with this:
  ```
  await db.profile.find_many(where={"NOT": [{"first_name": "null"}]})
  ```
  
  I don't know if this is the correct way. Because it would also filter a profile with the first_name "null".