When keyring replication makes a stale query to non-leader peers to find a key
the leader doesn't have, we need to make sure the peer we're querying has had a
chance to catch up to the most current index for that key. Otherwise it's
possible for newly-added servers to query another newly-added server and get a
non-error nil response for that key ID.

Note that the "not found" does not return an error, just an empty key. So as a
belt-and-suspenders, update the handling of empty responses so that we don't
break the loop early if we hit a server that doesn't have the key.