Fails when the relevant part of XSecController::exportOOXMLSignature()
is commented out, i.e. the member function just writes a <Signature> XML
element, no child elements.

Change-Id: I526c1170c8ada71f111a1ad56d74af1a4102dbce
Reviewed-on: https://gerrit.libreoffice.org/22375Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>