home-environment: make home.keyboard optional

When set to `null` then the `xsession` module will not attempt to
manage the keyboard settings.

modules/home-environment.nix

@@ -139,9 +139,12 @@ in
     };
 
     home.keyboard = mkOption {
-      type = keyboardSubModule;
+      type = types.nullOr keyboardSubModule;
       default = {};
-      description = "Keyboard configuration.";
+      description = ''
+        Keyboard configuration. Set to <literal>null</literal> to
+        disable Home Manager keyboard management.
+      '';
     };
 
     home.sessionVariables = mkOption {

modules/xsession.nix

@@ -66,39 +66,43 @@ in
   };
 
   config = mkIf cfg.enable {
-    systemd.user.services.setxkbmap = {
+    systemd.user = {
-      Unit = {
+      services = mkIf (config.home.keyboard != null) {
-        Description = "Set up keyboard in X";
+        setxkbmap =  {
-        After = [ "graphical-session-pre.target" ];
+          Unit = {
-        PartOf = [ "graphical-session.target" ];
+            Description = "Set up keyboard in X";
+            After = [ "graphical-session-pre.target" ];
+            PartOf = [ "graphical-session.target" ];
+          };
+
+          Install = {
+            WantedBy = [ "graphical-session.target" ];
+          };
+
+          Service = {
+            Type = "oneshot";
+            ExecStart =
+              let
+                args = concatStringsSep " " (
+                  [
+                    "-layout '${config.home.keyboard.layout}'"
+                    "-variant '${config.home.keyboard.variant}'"
+                  ] ++
+                  (map (v: "-option '${v}'") config.home.keyboard.options)
+                );
+              in
+                "${pkgs.xorg.setxkbmap}/bin/setxkbmap ${args}";
+          };
+        };
       };
 
-      Install = {
+      # A basic graphical session target for Home Manager.
-        WantedBy = [ "graphical-session.target" ];
+      targets.hm-graphical-session = {
-      };
+        Unit = {
-
+          Description = "Home Manager X session";
-      Service = {
+          Requires = [ "graphical-session-pre.target" ];
-        Type = "oneshot";
+          BindsTo = [ "graphical-session.target" ];
-        ExecStart =
+        };
-          let
-            args = concatStringsSep " " (
-              [
-                "-layout '${config.home.keyboard.layout}'"
-                "-variant '${config.home.keyboard.variant}'"
-              ] ++
-              (map (v: "-option '${v}'") config.home.keyboard.options)
-            );
-          in
-            "${pkgs.xorg.setxkbmap}/bin/setxkbmap ${args}";
-      };
-    };
-
-    # A basic graphical session target for Home Manager.
-    systemd.user.targets.hm-graphical-session = {
-      Unit = {
-        Description = "Home Manager X session";
-        Requires = [ "graphical-session-pre.target" ];
-        BindsTo = [ "graphical-session.target" ];
       };
     };