rgw_reshard: drop olh entries with empty name

rgw_reshard: drop olh entries with empty name

Due to a previous bug bucket indices can have olh entries with
an empty name. During reshard these are all mapped to shard 0,
easily triggering warnings about too many omap entries.

Skip these entries during reshard, thereby cleaning the bucket
of these leftover bogus entries.

Fixes: https://tracker.ceph.com/issues/54500
Related-to: https://tracker.ceph.com/issues/46456
Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>