org-attach-sync uses directory-empty-p (new in Emacs 28)

[Kyle Meyer]

Hi Marco,

In 61e083732 (org-attach: Possibly delete empty attach directory,
2021-07-09), you added a call to directory-empty-p.  This function was
introduced in Emacs's 0806075520 (Add directory-empty-p and new argument
COUNT for directory-files-*, 2020-11-02) and hasn't yet made it into a
release.

Could you update org-attach-sync to avoid using directory-empty-p (e.g.,
by inlining it or by adding a compatibility alias)?

Thanks.