rgwlc: fix lc head marker point to non-exist lc entry

rgwlc: fix lc head marker point to non-exist lc entry

After a single-bucket run and its lc entry is removed since the
bucket is deleted, we will have lc shard object's head marker
point to a non-exist lc entry. Finally, buckets that are hashed
to this lc shard can not have any lifecycle progress. Fix it
by skipping the non-exist marker.

Fixes: https://tracker.ceph.com/issues/57162
Signed-off-by: Tongliang Deng <dengtongliang@gmail.com>
(cherry picked from commit 479a9261670b14f058b280026f9cfcdf199badf8)