Even if the 'ad_actor_system' option is only valid for the 802.3ad
mode, the sysfs file is always present and has a default value of
''. But in 802.3ad mode the default value is
'00:00:00:00:00:00'. Return the correct value in
nm_setting_bond_get_option_default().

Furthermore, writing a empty string to the file will generate an
error, don't do it.