pkgs/switches/shared: fix for nullable vlan

nix/pkgs/switches/shared.nix

@@ -17,6 +17,13 @@ rec {
       else ra < rb
   );
   sortNetsByVlan = builtins.sort (net1: net2:
-    config.site.net.${net1}.vlan < config.site.net.${net2}.vlan
+    let
+      vlan1 = config.site.net.${net1}.vlan;
+      vlan2 = config.site.net.${net2}.vlan;
+    in if vlan1 == null
+       then true
+       else if vlan2 == null
+       then false
+       else vlan1 < vlan2
   );
 }