Replace strdup_or_null() by strdup_to()

I didn't know that this helper existed… It is very similar to strdup_to_full(),
but all callers can actually be replaced by strdup_to(), which has more fitting
semantics.

src/network/networkd-address.c

@@ -613,12 +613,12 @@ int address_dup(const Address *src, Address **ret) {
         dest->nft_set_context.n_sets = 0;
 
         if (src->family == AF_INET) {
-                r = strdup_or_null(src->label, &dest->label);
+                r = strdup_to(&dest->label, src->label);
                 if (r < 0)
                         return r;
         }
 
-        r = strdup_or_null(src->netlabel, &dest->netlabel);
+        r = strdup_to(&dest->netlabel, src->netlabel);
         if (r < 0)
                 return r;
 

src/network/networkd-route-metric.c

@@ -39,7 +39,7 @@ int route_metric_copy(const RouteMetric *src, RouteMetric *dest) {
         } else
                 dest->metrics_set = NULL;
 
-        return strdup_or_null(src->tcp_congestion_control_algo, &dest->tcp_congestion_control_algo);
+        return strdup_to(&dest->tcp_congestion_control_algo, src->tcp_congestion_control_algo);
 }
 
 void route_metric_hash_func(const RouteMetric *metric, struct siphash *state) {

src/network/networkd-route-nexthop.c

@@ -218,7 +218,7 @@ static int route_nexthop_copy(const RouteNextHop *src, RouteNextHop *dest) {
         /* unset pointer copied in the above. */
         dest->ifname = NULL;
 
-        return strdup_or_null(src->ifindex > 0 ? NULL : src->ifname, &dest->ifname);
+        return strdup_to(&dest->ifname, src->ifindex > 0 ? NULL : src->ifname);
 }
 
 static int route_nexthop_dup(const RouteNextHop *src, RouteNextHop **ret) {