handle_handshake exceptions out on odd username circumstances

This happened on windows but not linux (couldn't tell why). When in a skip_database scenario and not passing the unneeded opt username, the protocol.ex line in handle hanshake :unicode.characters_to_binary(opts[:username]) errors out (presumably on nil) with an argument error. Passing the opt username: "" fixed it.

Not urgent, I can put a PR up next week if you like, to address it