RepositoryEmpty isn't actually empty

[kanzure]

This isn't an "issue" so much as a note I'd like to leave for others. Maybe a code change is warranted or a README update?

RepositoryEmpty.__contains__ is correctly implemented but __getitem__ returns None, which does not seem like an appropriate default value for app environment variables. Collections like a dictionary's __getitem__ will raise a KeyError instead.

Another alternative for an actually empty repository is to pass an empty dictionary instead of using RepositoryEmpty.

I don't know why RepositoryEmpty exists.