Cope with deleted users

If the user has been deleted since they were selected by ACF then we get a bunch of errors.  This change checks they exist before trying to use them.

Calling code then has to cope with the missing data e.g.

        $users = get_field('users');

        foreach($users as $user){

                if(!isset($user) || $user == null || !is_array($user)){
                  continue; // Or print something saying the user has been deleted
                }

                // do something with the user
        }