It is possible for a row in Cassandra to have a missing metric_id
(shows as 'null' in cqlsh). This causes an ugly NoneType error
to be passed up to the user on the command line for
'monaca metric-list'.
Fix is to detect the missing value, log an error, and return the
row with None for the metric_id.
Change-Id: Ie617932c6b12a6cfe441510e120bb77a3470b9cf
Story: 2005305
Task: 30194