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

[Eli Zaretskii]

> From: martin rudalics <rudalics@gmx.at>
> Date: Fri, 14 May 2021 09:15:09 +0200
> 
>  >  > In package windows.el, function `window-default-font-height' calls
>  >  > function `face-font', which can return a nil value, but performs a
>  >  > `string-equal' operation on the output without checking for that
>  >  > possibiliy.
>  >
>  > Please report this to the author(s) of that package.
> 
> 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...