Key endpoints returns 400 when there aren't any keys

[DomAyre]

This applies to a couple of endpoints /listpubkeys, /key and pubkey, for /listpubkeys not sure if this is intended behaviour or not, but I would assume we should return 200 with the same schema as when there are keys, with an empty list of keys, but instead we get nothing and a return of 400

[DomAyre]

Looking at the implementation it looks like this is deliberate, but I'm going to keep the issue open because I wonder if it would be better to return 200 with an empty list or a 404