bug#43375: 26.1: file-directory-p returns t for empty string

[Eli Zaretskii]

> Date: Sun, 13 Sep 2020 12:29:00 -0400
> From: Boruch Baum <boruch_baum@gmx.com>
> Cc: schwab@linux-m68k.org, 43375@debbugs.gnu.org
> 
> > I'm okay with making the doc string more clear, but let's please think
> > about a more useful amendment.  A doc string should help the user
> > understand what will/did happen, it shouldn't present puzzles.
> 
> How about:
> 
>   Return t if evaluating `expand-file-name' on FILENAME names an existing
>   directory.

How does that help the reader predict what will happen in each and
every case without actually trying?

Anyway, I think this is a moot point, since Lars already fixed the doc
string to mention this special case, and the current text is clear
enough to my palate.