CVE-2022-3854: rgw: Fix `rgw::sal::Bucket::empty` static method signatures

CVE-2022-3854: rgw: Fix `rgw::sal::Bucket::empty` static method signatures

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
Fixes: https://tracker.ceph.com/issues/56586
Origin: upstream, https://github.com/ceph/ceph/pull/47194/commits/9746e8011ff1de6de7dba9c0041e28a16c8f6828.patch
Bug-Debian: https://bugs.debian.org/1027151
Last-Update: 2022-01-09

`unique_ptr` overload should take by reference.

Both should be const.

Gbp-Pq: Name CVE-2022-3854_2_rgw_Fix_rgw_sal_Bucket_empty_static_method_signatures.patch