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

Change-Id: I52e5135139fdf88abe380b6e5ae60e2afb582289
Reviewed-on: https://gerrit.libreoffice.org/7556
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>