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

Change-Id: I25d7cbd260384110173fe953fc24f3dcf6b9acd5
Reviewed-on: https://gerrit.libreoffice.org/37770Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>