Add changelog for preloading project

changelogs/unreleased/39113-project-user-grafana-integrations.yml

+---
+title: Preload project, user and group to reuse objects
+merge_request: 21853
+author:
+type: performance

lib/gitlab/import_export/relation_factory.rb

@@ -131,6 +131,8 @@ module Gitlab
       end
 
       def preload_keys(object, references, value)
+        return object unless value
+
         references.each do |key|
           attribute = key.delete_suffix('_id').to_sym
           next unless object.respond_to?(key) && object.respond_to?(attribute)