services.xserver: fix `apply` of default DM/WM

This is needed to pick the first enabled DM/WM
if the default is `none`

nixos/modules/services/x11/desktop-managers/default.nix

@@ -91,7 +91,7 @@ in
         example = "plasma5";
         description = "Default desktop manager loaded if none have been chosen.";
         apply = defaultDM:
-          if defaultDM == "" && cfg.session.list != [] then
+          if defaultDM == "none" && cfg.session.list != [] then
             (head cfg.session.list).name
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM

nixos/modules/services/x11/window-managers/default.nix

@@ -61,7 +61,9 @@ in
         example = "wmii";
         description = "Default window manager loaded if none have been chosen.";
         apply = defaultWM:
-          if any (w: w.name == defaultWM) cfg.session then
+          if defaultWM == "none" && cfg.session != []  then
+            (head cfg.session).name
+          else if any (w: w.name == defaultWM) cfg.session then
             defaultWM
           else
             throw "Default window manager (${defaultWM}) not found.";

nixos/modules/services/x11/xserver.nix

@@ -562,7 +562,9 @@ in
                 + concatMapStringsSep ", " (x: x.output) primaryHeads;
       })
       { assertion = cfg.desktopManager.default == "none" && cfg.windowManager.default == "none" -> cfg.plainX;
-        message = "Either the desktop manager or the window manager shouldn't be `none`! To explicitly allow this, you can also set `services.xserver.plainX` to `true`.";
+        message = "Either the desktop manager or the window manager shouldn't be `none`! "
+                + "To explicitly allow this, you can also set `services.xserver.plainX` to `true`. "
+                + "The `default` value looks for enabled WMs/DMs and select the first one.";
       }
     ];