Scylla metadata could be empty due to bugs like the one introduced by
115ff10. Let's make shard computation resilient to empty sharding
metadata by falling back to the approach that uses first and last
keys to compute shards.
Refs #2932.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20171223120140.3642-2-raphaelsc@scylladb.com>