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

[Eli Zaretskii]

> Cc: boruch_baum@gmx.com, 48408@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sun, 16 May 2021 11:28:00 +0200
> 
>  > 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.
> 
> Is it possible that `window-font-width' and `window-font-height' run
> into a similar problem when `face-font' returns nil?

I don't think so, because they call display-multi-font-p without
arguments.