lib/openwrt-models: work around empty lines

nix/lib/openwrt-models.nix

@@ -28,8 +28,14 @@ let
         builtins.filter (word:
           word != [] && word != ""
         ) tokens;
-      command = builtins.head words;
-      args = builtins.tail words;
+      command =
+        if words != []
+        then builtins.head words
+        else "-";
+      args =
+        if words != []
+        then builtins.tail words
+        else [];
 
       makeLinkFromArg = port: arg:
         builtins.foldl' (result: interface: