Improve API handling for 'none' delivery-channel

Extend the [`none`](https://dlcs.github.io/public-docs/api-doc/delivery-channels/#the-none-channel) channel handling in the API. This channel  is already supported; assets that have the `none` channel are handled and never served. 

The requirement for this originates in https://github.com/dlcs/protagonist/blob/develop/docs/rfcs/022-stub-assets.md#assets-without-content