key import with an empty password doesn't work

i did a `mix nerves_hub.key import keyname /path/to/key.pub /path/to/key.priv` and just mashed `enter` until it errored out, but even tho there was an error, it actually imported the key. when i did `mix nerves_hub.key delete` and mashed enter it gave an error, and didn't delete it. 

I'm fine with not allowing empty password, but right now it gets left in a limbo state if you try this. 