resolved fdo#70275 write correct office:string-value if text format applied

Change-Id: I52e5135139fdf88abe380b6e5ae60e2afb582289
Reviewed-on: https://gerrit.libreoffice.org/7555
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>