fdo#69616: Fix for VML part missing for group.

Cause:
 - In altenrate content, Fallback contains only group tag.
Implementation:
 - Added export logic in Vml export.
 - Added unit test case for vml group.

Change-Id: Ia1c9834950528dc892caea1cb675a7f42165d9ba
Reviewed-on: https://gerrit.libreoffice.org/7276
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>