radosgw-admin: 'sync status' is not behind if there are no mdlog entries

radosgw-admin: 'sync status' is not behind if there are no mdlog entries

if remote mdlogs are trimmed prematurely, sync status will report
that it's behind the remote's max-marker even if there are no mdlog
entries to sync

for each behind shard, we fetch the next mdlog entry from the remote. if
we get an empty listing, remove that shard from behind_shards. this
logic now has to run before we print "behind shards:" so that empty
shards aren't listed

Fixes: https://tracker.ceph.com/issues/52091
Signed-off-by: Casey Bodley <cbodley@redhat.com>