Fix searching for members without a role

I have noticed, that the index page of Postorius sometimes displays mailing lists you have no association with, meaning you're neither a subscriber, moderator nor owner. This mostly happens when you are part of many mailing lists or if you'r a superuser. The way I understand it, find_members should not return such lists when no role is given. This change will make sure that find_members only returns lists the user is actually a subscriber, moderator or owner of when no role is given.