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

[Eli Zaretskii]

> Cc: 48408@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sun, 16 May 2021 10:35:31 +0200
> 
>  > Martin, same question about this one.  I decided that there's nothing
>  > wrong with how we compute frame-type in the display-*-p predicates;
>  > it's window-default-font-height that trusts the 'display' frame
>  > parameter too much.  WDYT?
> 
> OK.  But was Boruch's original idea just checking for
> 
> (stringp default-font)
> 
> worse?

I'm asking for your opinion about that ;-)