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

[Eli Zaretskii]

> Date: Wed, 19 May 2021 00:00:53 -0400
> From: Boruch Baum <boruch_baum@gmx.com>
> Cc: martin rudalics <rudalics@gmx.at>, 48408@debbugs.gnu.org
> 
> On 2021-05-16 11:30, Eli Zaretskii wrote:
> > Which is a Good Thing, because I've changed my mind.  Please try the
> > much simpler patch below.
> > ...
> > -    (if (and (display-multi-font-p (frame-parameter frame 'display))
> > +    (if (and (not (eq (framep frame) t))
> > +             (display-multi-font-p (frame-parameter frame 'display))
> 
> This patch does do the job; The bug does not happen with this patch.

Thanks, installed.