Option to encode nil slice/map as []/{}

Given that:

1. Empty slices/maps are often represented as `nil` in go (i.e., go doesn't really care).
2. Most *other* languages distinguish between `null` and an empty list/object.

It would be nice to have an option to always encode empty slices/maps as `[]`/`{}`.

Unfortunately, doing this with an atlas doesn't appear to be possible as maps/slices are generic.