radosgw-admin: 'bi purge' doesn't require entrypoint

we may get -ENOENT looking up cur_bucket here. we look up cur_bucket so
we can avoid purging the 'current' bucket instance. but if that
entrypoint doesn't exist, there is no current instance and that
shouldn't prevent us from purging

Fixes: https://tracker.ceph.com/issues/52976
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 94db3409178ad6986609fd9c4dd9b152a4722f7c)

Conflicts:
  - path src/rgw/rgw_admin.cc
    comment: manually resolve conflicts