m-default-routes: return if default routes for a device are not found

Avoids unneeded assertion error messages.
2 jobs for lua-object-improvements in 1 minute and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container
passed #7991931
container_fedora

00:00:23

 
  Build
passed #7991932
build_on_fedora

00:00:43