keyboard: make the model option optional

Also, actually use it in the call to setxkbmap.

modules/home-environment.nix

@@ -71,8 +71,8 @@ let
       };
 
       model = mkOption {
-        type = types.str;
-        default = "pc104";
+        type = with types; nullOr str;
+        default = null;
         example = "presario";
         description = ''
           Keyboard model.

modules/xsession.nix

@@ -109,6 +109,7 @@ in
                 args =
                   optional (layout != null) "-layout '${layout}'"
                   ++ optional (variant != null) "-variant '${variant}'"
+                  ++ optional (model != null) "-model '${model}'"
                   ++ map (v: "-option '${v}'") options;
               in
                 "${pkgs.xorg.setxkbmap}/bin/setxkbmap ${toString args}";