Assets: disable automatic preview generation for node groups

The current preview generation is more confusing than useful.
Therefore it is better to disable it until better preview generation
methods are found.

Differential Revision: https://developer.blender.org/D13728

source/blender/editors/render/render_preview.cc

@@ -1703,7 +1703,10 @@ bool ED_preview_id_is_supported(const ID *id)
   if (id == nullptr) {
     return false;
   }
-
+  if (GS(id->name) == ID_NT) {
+    /* Node groups don't support standard preview generation. */
+    return false;
+  }
   if (GS(id->name) == ID_OB) {
     return object_preview_is_type_supported((const Object *)id);
   }