The handler for nvGrpSpPr is "this class", not "no class".

Change-Id: I7fc40681d77f94b7473d2f677d813d2ae246f6ac
Reviewed-on: https://gerrit.libreoffice.org/40854Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>