bug#64759: Broken faces

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: maurooaranda@gmail.com,  64759@debbugs.gnu.org
> Date: Fri, 21 Jul 2023 21:49:13 +0300
> 
> No, it's not the same.  The latter uses a white background on the mode line.

So I ask again: what is the semantics of nil in this case?  We cannot
have in Emacs something that triggers undefined behavior, let alone
use it in our own sources.  The semantics of this must be known and
documented before we can use it.