Config plugin: return EmptyModlist when no change is applied
    
    When ipa config-mod is called with the option --enable-sid,
    the code needs to trap EmptyModlist exception (it is expected
    that no LDAP attribute is modified by this operation).
    The code had a flaw and was checking:
        'enable_sid' in options
    instead of
        options['enable_sid']
    
    "'enable_sid' in options" always returns true as this option
    is a Flag with a default value, hence always present even if
    not specified on the command line.
    
    Fixes: https://pagure.io/freeipa/issue/9063
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>