As pointed out via DOVE-Net by DesotoFireflite (VALHALLA):
When xtrn_sec.* display file/menu is used, there's no auto-generated
prompt (with a default section number indicated). So hitting Enter
in this case and having the previous/default program section selected
would be surprising behavior to a user. So, only support a default
section selection when there was no xtrn_sec.* menu/display file.