This bug caused signature line IDs in OOXML to be written
with an appended "_x0000_".

Change-Id: If219279a5d5177ec5fac2d9dd747dc015c16b6bc
Reviewed-on: https://gerrit.libreoffice.org/60736Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>