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

[Eli Zaretskii]

> Date: Sat, 15 May 2021 23:06:18 -0400
> From: Boruch Baum <boruch_baum@gmx.com>
> Cc: 48408@debbugs.gnu.org
> 
> >   M-: (frame-parameter nil 'display) RET
> 
> ":0.0"

That's the cause of the problem: it should say nil instead.

Can you step through framep-on-display in that case and tell what does
each of the expressions in 'or' there yield for that frame?  That is,
each one of these:

  (terminal-live-p ":0.0")
  (framep ":0.0")
  (frames-on-display-list ":0.0")

My bet is that the 3rd one produces the bad result.

Thanks.