Details element displayed despite child entity reference field being empty

Problem/Motivation

I have a multi-value entity reference field that I've placed in a 'details' element via Field Group. This works exactly as intended, except when the entity reference field is empty. The 'details' element is still displayed, and when opened, shows and empty div.

This is unexpected (and undesirable), since the "Display element also when empty" option is unchecked.

Proposed resolution

Check child fields for empty item lists in field_group_remove_empty_display_groups()