`NullRelation` has been nodoc-ed [here](https://github.com/rails/rails/commit/30bf42b3ef8be104abffba07c3feaa343d882e09), so I think we should avoid mentioning it altogether in the docs.

Perhaps "The returned relation implements the Null Object pattern. When converted to an array, it will return an empty array without querying the database.".

You can iterate on this, I'm not 100% happy with that, but the important thing is to remove the `NullRelation` mention. (The `defined null behavior` bugs me a little bit, because it seems overly technical for the average reader and doesn't provide much benefit, but it's not a very big deal.)