osd/ReplicatedPG: handle missing hit_set on HITSET_GET rados op

Fixes: #8081
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1660 from ceph/wip-hitset-missing

osd: handle hitset-get on a missing hit_set object

Reviewed-by: Samuel Just <sam.just@inktank.com>