bug#40576: call-process-region does not accept nil as first argument

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sun, 12 Apr 2020 19:37:07 +0200
> Cc: Pietro Giorgianni <giorgian@gmail.com>, 40576@debbugs.gnu.org
> 
> > But the restrictions aren't ignored.  They are only ignored if START
> > is nil.  That makes no sense.
> 
> I completely agree that it doesn't make sense. I'm just saying that
> the behavior has been there for so long that we realistically can't
> change it any more.

I think we can change it, but I won't argue further.