Leaving Signer as an empty reference will do exactly what we want: the
signature will be considered invalid.

(cherry picked from commit 18aa83ac)

Change-Id: I25d7cbd260384110173fe953fc24f3dcf6b9acd5
Reviewed-on: https://gerrit.libreoffice.org/37813Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit 398ab416)