bug#48408: BUGFIX: window-default-font-height: check for nil string

[Eli Zaretskii]

> Date: Sun, 16 May 2021 11:56:45 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: boruch_baum@gmx.com, 48408@debbugs.gnu.org
> 
> > OK.  But was Boruch's original idea just checking for
> > 
> > (stringp default-font)
> > 
> > worse?
> 
> I'm asking for your opinion about that ;-)

Actually, no, I do have an opinion on that: face-font can return a
string even on a TTY frame, at least in a build --without-x.  (Which
seems to be an inconsistency in and of itself, btw.)

In general, trusting frame parameters too much is IME not a good idea,
as they are too loosely defined.