ScDocument already has one sheet upon creation, so we need to avoid
appending sheet when the request is for the first sheet.

Change-Id: Ie5526d6791e545f5faa0bf3f16988351c09e94c8
Reviewed-on: https://gerrit.libreoffice.org/46658Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>