Fix: Missing early return when asset isn't found

This didn't cause a crash since `resolve_asset_weak_reference_to_full_path`
doesn't actually use the `this` pointer, but it's still quite unsafe.

source/blender/editors/asset/intern/asset_menu_utils.cc

@@ -103,6 +103,7 @@ static const asset_system::AssetRepresentation *find_asset_from_weak_ref(
       asset_system::all_library_reference());
   if (!all_library) {
     BKE_report(reports, RPT_WARNING, "Asset loading is unfinished");
+    return nullptr;
   }
 
   const std::string full_path = all_library->resolve_asset_weak_reference_to_full_path(weak_ref);