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.
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