A while back the way that backgrounds are handled changed.
This was one spot where the changes didn't sync up. When
the background is cleared, then the imported grabbag
attributes should be tossed, not re-applied during save.

Change-Id: I9759def6321697a634bcbd66ecdc9d73693ce1a3
Reviewed-on: https://gerrit.libreoffice.org/44040Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>