Fix for datedone sorting if there is no date done for a torrent

For rtorrent at least, date done isn't a normal field and we get it in transdroid from the seedingtime ruTorrent plugin. If this plugin wasn't installed from day 1 then existing torrents won't have a date done set and so they will all end up at the top of a date descending list. This change makes sure that any complete torrent without a datedone field get sorted at the bottom of the list list