"Nil" CIDs

[Stebalien]

We're now using cid.Cid{} (empty) as the "nil" CID. Unfortunately, refmt won't encode this to null. With polydawn/refmt#36, we can at least omit empty CIDs however, we still need to be able to do something intelligent with empty/nil CIDs.

• We could just panic or return an error but, IMO, we should do something a bit smarter.
• We can also inject a token "mapper" to transform the token stream. It may be worth adding this feature to atlas entries?