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

[martin rudalics]

> 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?

martin