Bugfix: Treat an empty password as a fatal error for repository init

When attempting to initialize a new repository, if an empty password was
supplied, the repository would be created but the init command would return
an error with a stack trace. Now, if an empty password is provided, it is
treated as a fatal error, and no repository is created.

https://github.com/restic/restic/issues/3214
https://github.com/restic/restic/pull/3283
