services.xserver: assert that either desktop- or window manager is not "none"

resolves #11064

nixos/doc/manual/release-notes/rl-1803.xml

@@ -90,6 +90,14 @@ following incompatible changes:</para>
       That means that old configuration is not overwritten by default when update to the znc options are made.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The option <option>services.xserver.desktopManager.default</option> is now <literal>none</literal> by default.
+      An assertion failure is thrown if WM's and DM's default are <literal>none</literal>.
+      To explicitly run a plain X session without and DM or WM, the newly introduced option <option>services.xserver.plainX</option>
+      must be set to true.
+    </para>
+  </listitem>
 </itemizedlist>
 
 </section>

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

@@ -87,8 +87,8 @@ in
 
       default = mkOption {
         type = types.str;
-        default = "";
-        example = "none";
+        default = "none";
+        example = "plasma5";
         description = "Default desktop manager loaded if none have been chosen.";
         apply = defaultDM:
           if defaultDM == "" && cfg.session.list != [] then

nixos/modules/services/x11/xserver.nix

@@ -161,6 +161,15 @@ in
         '';
       };
 
+      plainX = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether the X11 session can be plain (without DM/WM) and
+          the Xsession script will be used as fallback or not.
+        '';
+      };
+
       autorun = mkOption {
         type = types.bool;
         default = true;
@@ -552,6 +561,9 @@ in
                 + "${toString (length primaryHeads)} heads set to primary: "
                 + 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`.";
+      }
     ];
 
     environment.etc =