resolved rhbz#1101224 do not attempt to obtain names for NULL tabs

This happened when the HTML export via clipboard tried to resolve
conditional formats, where we have a temporary instance of a document
containing only the sheet to be exported.

Change-Id: Ic7498a1cab3eabede74773868287a2cc3edef052
(cherry picked from commit 17979abf4fde202cae231be19a218be3fe27d04c)
Reviewed-on: https://gerrit.libreoffice.org/9506
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>