rgw: Fix `rgw::sal::Bucket::empty` static method signatures

rgw: Fix `rgw::sal::Bucket::empty` static method signatures

`unique_ptr` overload should take by reference.

Both should be const.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit b1d3e6c00674ebf6bde08968789a426d65db73d9)

Conflicts:
	src/rgw/rgw_sal.h
 - `unique_ptr` overload of empty

Fixes: https://tracker.ceph.com/issues/56585
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit e40ce4a3511e669e761da5f39d81b14e6cdbdeba)
Resolves: rhbz#118423
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>