state-routes: use get_count() instead of next()

We cannot use next() as the properties are not a Lua table anymore.

src/scripts/device/state-routes.lua

@@ -114,7 +114,7 @@ apply_route_props_hook = SimpleEventHook {
     local dev_info = devinfo:get_device_info (device)
     assert (dev_info)
 
-    if next (selected_routes) == nil then
+    if selected_routes:get_count () == 0 then
       log:info (device, "No routes selected to set on " .. dev_info.name)
       return
     end