Geometry Nodes: Show hint in empty output attributes panel

This is meant to add something to the sub-panel when it is empty
so it looks more purposeful, but also add a hint that might be helpful
when figuring out how to output a named attribute.

Differential Revision: https://developer.blender.org/D12715

source/blender/modifiers/intern/MOD_nodes.cc

@@ -1428,13 +1428,18 @@ static void output_attribute_panel_draw(const bContext *UNUSED(C), Panel *panel)
   uiLayoutSetPropSep(layout, true);
   uiLayoutSetPropDecorate(layout, true);
 
+  bool has_output_attribute = false;
   if (nmd->node_group != nullptr && nmd->settings.properties != nullptr) {
     LISTBASE_FOREACH (bNodeSocket *, socket, &nmd->node_group->outputs) {
       if (socket_type_has_attribute_toggle(*socket)) {
+        has_output_attribute = true;
         draw_property_for_output_socket(layout, *nmd, ptr, *socket);
       }
     }
   }
+  if (!has_output_attribute) {
+    uiItemL(layout, IFACE_("No group output attributes connected."), ICON_INFO);
+  }
 }
 
 static void panelRegister(ARegionType *region_type)