rgw/lc: Fix issues with non-current objects with instance empty

rgw/lc: Fix issues with non-current objects with instance empty

When the bucket versioning is enabled, old plain object entry is converted
to versioned by updating its instance as "null" in its raw head/old object.
However its instance remains empty in the bi list entry. Same is the case for
the entries created after versioning is suspended and re-enabled.

So to access such objects which are non-current, we need to set rgw_obj_key.instance as
1) "null" to read the actual raw obj and
2) empty while accessing/updating their bi entry.

Fixes: https://tracker.ceph.com/issues/68402
Signed-off-by: Soumya Koduri <skoduri@redhat.com>