Epic board positioning doesn't work for closed lists

In !59379 (merged) we added support for repositioning epics on epic boards, but this doesn't work as expected when moving an epic in the closed list - in that case the request fails.

The reason is that the SQL query for filling missing positions doesn't contain JOIN with relative positions table because this JOIN is added in ordered_items method which is skipped for closed lists - https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/boards/base_items_list_service.rb#L9