if conf unset, don't read / as conf/pref/sources dir

if conf unset, don't read / as conf/pref/sources dir

Usually these config options are set to sensible values, but if init
isn't run or the user interferes with configuration clearing or similar
the options could indeed carry an empty value, which will result in
FindDir returning a '/'. That feels kinda wrong, but as a public
interface there isn't much we can do about it and instead make it so
that we get the special file /dev/null back we know how to deal with in
such cases.