rgw: if extra data pool name is empty, use data pool name instead

Fixes: #8311
An empty pool name could be used as the extra data pool.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1907 from ceph/wip-8311

rgw: if extra data pool name is empty, use data pool name instead

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>