fix(study): correct access to study additional_data (#1949)

Avoid `Attribut Error` when a study has no `additional_data`.

antarest/study/storage/utils.py

@@ -52,7 +52,7 @@ def update_antares_info(metadata: Study, study_tree: FileStudyTree, *, update_au
     study_data_info["antares"]["created"] = metadata.created_at.timestamp()
     study_data_info["antares"]["lastsave"] = metadata.updated_at.timestamp()
     study_data_info["antares"]["version"] = metadata.version
-    if update_author:
+    if update_author and metadata.additional_data:
         study_data_info["antares"]["author"] = metadata.additional_data.author
     study_tree.save(study_data_info, ["study"])