protect against trying to search for empty metric.Id.
protect against trying to search for empty metric.Id.

- trying to search for an id that is an empty string will lead
  to a fairy cryptic error from Elasticsearch.

raintank/metrictank,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/raintank-metric,OOM-Killer/metrictank,raintank/metrictank,replay/metrictank,OOM-Killer/metrictank,replay/metrictank,raintank/metrictank,raintank/raintank-metric,OOM-Killer/metrictank