bug#46184: 28.0.50; child-frame-border-width of 0 falls back to internla-border-width

[Alexander Miller]

> > (make-frame `(
> > (parent-frame . ,(window-frame))
> > (child-frame-border-width . 0)
> > (internal-border-width . 200)))
> >
> > I would expect the border width to only fall back if
> > child-frame-border-width is nil, since 0 is a useful setting.
>
> It's not entirely trivial to do that (frame parameters are a pain in
> this regard) but please have a look at the attached patch (tested on a
> GTK build only). Alexander, please have a look too.
>
> Thanks, martin

I have tried the patch and the case for using 0 does work without
fallback now.

I also tried setting an explicit `nil` value and got an error, but
judging by how `internal-border-width` shows the same behaviour I assume
that it's working as expected.