Import group with ndjson only if needed

app/services/groups/import_export/import_service.rb

@@ -36,7 +36,7 @@ module Groups
 
       def restorer
         @restorer ||=
-          if ::Feature.enabled?(:group_import_export_ndjson, @group&.parent)
+          if ndjson?
             Gitlab::ImportExport::Group::TreeRestorer.new(
               user: @current_user,
               shared: @shared,
@@ -52,6 +52,11 @@ module Groups
           end
       end
 
+      def ndjson?
+        ::Feature.enabled?(:group_import_export_ndjson, @group&.parent) &&
+          File.exist?(File.join(@shared.export_path, 'tree/groups/_all.ndjson'))
+      end
+
       def remove_import_file
         upload = @group.import_export_upload