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: Fri, 14 May 2021 10:15:58 +0200
> 
>  >> Sorry.  This function is missing from my current window.el here ...
>  >
>  > Really?  It is still there on today's master.  So I wonder what kind
>  > of revolution do you have waiting in the wings...
> 
> Nothing special.  Just a function that takes its buffer's face remapping
> into account.  But it lacks an alias and is apparently just as broken as
> the old one, see below.

Yes, the same problem should affect this version as well, since it
still assumes (correctly) that if the frame supports fonts, the
default font's name is a string.