Fix exit status

[schultyy]

When there are no actions run, there are no exit codes but Sokan exits with code 1 because of:

    //src/shell.rs
    exit_codes.sort();

    match exit_codes.last() {
        Some(&0) => return 0,
        _       => return 1
    }

last returns probably None and defaults to 1. But instead it should return with exit status 0.