Fixes https://github.com/dhall-lang/dhall-kubernetes/issues/86
The motivation of this is to more accurately model the Kubernetes
API semantics by not auto-omitting empty fields. This is because
a field set to `Some ([] : List T)` is not necessarily the same
as `None (List T)`.
This makes the typical case a bit more verbose (more `Some`s), but
the difference to the `./examples` shows that it's not too bad.
