--create-role should allow empty role descriptions

[jeanlucthumm]

This came up in #557

Right now if you do sgpt --create-role empty and you just Enter on this prompt:

Enter role description:

It doesn't let you go through.

If you write "" then it creates a role like this which is also bad:

{"name": "empty", "role": "You are empty\n\"\""}

Big problem though, markdown output is dependent on whether the words APPLY MARKDOWN are in the role, so markdown enabled completely empty roles are not possible.