There is always a section break at the end of a DOCX document, so in the
rare case when there is no text node between the end of a Writer section
and the end of the document make sure no duplicated section break is
written.

Change-Id: I3fcd852c1a63e2d707822ad08603e2d37386e439
Reviewed-on: https://gerrit.libreoffice.org/35499Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>