Handle empty netns in DEL for loopback and host-device.

Signed-off-by: Lantao Liu <lantaol@google.com>

plugins/main/host-device/host-device.go

@@ -85,6 +85,9 @@ func cmdDel(args *skel.CmdArgs) error {
 	if err != nil {
 		return err
 	}
+	if args.Netns == "" {
+		return nil
+	}
 	containerNs, err := ns.GetNS(args.Netns)
 	if err != nil {
 		return fmt.Errorf("failed to open netns %q: %v", args.Netns, err)