passphrase cannot be blank

[lionbee]

Perhaps this is broken by design, but if you try to decrypt a key with an empty passphrase bcrypt_pbkdf will return -1 indicating an error occurred.

https://github.com/joyent/node-bcrypt-pbkdf/blob/9ae4b4912099b2dc2ff38ab34934324523ab8025/index.js#L508-L510