scanning devices for legacy os-* interface is broken

because legacy_smart_interface::scan_smart_devices is called with type==NULL
and get_ata_device(atanames[i], type) called within it abends because of
such NULL:

Smartd: Exception: basic_string::_S_construct NULL not valid

As I workaround the NULL type is set to ""


git-svn-id: https://smartmontools.svn.sourceforge.net/svnroot/smartmontools/trunk@2687 4ea69e1a-61f1-4043-bf83-b5c94c648137