firstboot: fix root params with creds and prompting disabled

firstboot: fix root params with creds and prompting disabled

Remove an early return that prevents --prompt-root-password or
--prompt-root-shell and systemd.firstboot=off using credentials. In that case,
arg_prompt_root_password and arg_prompt_root_shell will be false, but the
prompt helpers still need to be called to read the credentials. Furthermore, if
only the root shell has been set, don't overwrite the root password.

(cherry picked from commit 35bc4c34240afdd55e117b909f26fa9a5dc54f3b)