UI: Use "unassigned" icon for assets in modifier and node add menus

Use the same icon as in the asset browser "unassigned" item for
unassigned node group assets exposed in the modifier or node
add menus, helping to make the connection to the asset browser.
The downside is potentially drawing unnecessary attention to these
categories, but ideally they won't exist for most users anyway (since
the assets should be in proper catalogs).

source/blender/editors/object/add_modifier_assets.cc

@@ -195,7 +195,10 @@ static void root_catalogs_draw(const bContext *C, Menu *menu)
 
   if (!tree.unassigned_assets.is_empty()) {
     uiItemS(layout);
-    uiItemM(layout, "OBJECT_MT_add_modifier_unassigned_assets", IFACE_("Unassigned"), ICON_NONE);
+    uiItemM(layout,
+            "OBJECT_MT_add_modifier_unassigned_assets",
+            IFACE_("Unassigned"),
+            ICON_FILE_HIDDEN);
   }
 }
 

source/blender/editors/space_node/add_menu_assets.cc

@@ -216,7 +216,7 @@ static void add_root_catalogs_draw(const bContext *C, Menu *menu)
 
   if (!tree.unassigned_assets.is_empty()) {
     uiItemS(layout);
-    uiItemM(layout, "NODE_MT_node_add_unassigned_assets", IFACE_("Unassigned"), ICON_NONE);
+    uiItemM(layout, "NODE_MT_node_add_unassigned_assets", IFACE_("Unassigned"), ICON_FILE_HIDDEN);
   }
 }