sd-network: when the LLMNR setting is not known for an interface consider that as ENODATA

ENODATA should be returned whenever we have no idea about something. A
missing LLMNR setting can only really happen during upgrades, in whichc
ase we really have no idea, so let's turn this into another ENODATA
case.

src/network/sd-network.c

@@ -122,6 +122,8 @@ _public_ int sd_network_get_llmnr(int ifindex, char **llmnr) {
                 return -ENODATA;
         else if (r < 0)
                 return r;
+        else if (!s)
+                return -ENODATA;
 
         *llmnr = s;
         s = NULL;