Also provide a default pivot table style for those tables that don't
have a style info. Let's use the style settings that Excel uses.

Change-Id: I8006a33a0aa0e92629f7db0a9c24a6ff52d17945
Reviewed-on: https://gerrit.libreoffice.org/70933
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>