s3 allows for zero sized objects, which can be
created using chunked uploads. It is important to
call "recv_body (and ChunkMeta::create_next()) even
on zero sized objects, so that the proper chunk signature
can be presented at signature verification time.
Fixes: http://tracker.ceph.com/issues/21000 Signed-off-by: Marcus Watts <mwatts@redhat.com>