libnm: change empty-GPtrArray-return semantics

libnm: change empty-GPtrArray-return semantics

libnm functions that return GPtrArrays of objects had a rule that if
the array was empty, they would return NULL rather than a 0-length
array. As it turns out, this is just a nuisance to clients, since in
most places the code for the non-empty case would end up doing the
right thing for the empty case as well (and where it doesn't, we can
check "array->len == 0" just as easily as "array == NULL"). So just
return the 0-length array instead.