I know this question has been asked a while ago but I’d drop my answer which might help someone out there who is having same issue.
The reason why you got the ERROR: The password field is empty. is because the crediential array key for the password is incorrect. In your code you have the key as user_pass
but it needs to be user_password
as stated here.
This portion of your code
$creds = array(
'user_login' => $user->user_login,
'user_pass' => $user_pass_login
);
needs to be:
$creds = array(
'user_login' => $user->user_login,
'user_password' => $user_pass_login
);