[Exim] empty senders address list

I block unknown senders with this director:

verify_known_sender:
driver = smartuser
require_files = /home/$local_part/.mail-accept-list
senders = ! /home/$local_part/.mail-accept-list
new_address = :fail: Sender unknown

But if .mail-accept-list file is empty then it doesn't "fail" (so anyone
can send to the user).

I understand if the last item is negated then it behaves like a wildcard
is at end.

I want it to fail if the senders is empty or does not match one of the
email address in this .mail-accept-list file.

Any ideas?

Thanks,

Jeremy C. Reed