fix(storage): Add unit tests to verify nil cursor
fix(storage): Add unit tests to verify nil cursor

When matching series have no data, the NewGroupResultSet cursor should
return nil. Added tests to verify these conditions.

influxdb/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdb/influxdb,nooproblem/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,influxdata/influxdb,nooproblem/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,li-ang/influxdb,influxdb/influxdb,nooproblem/influxdb,nooproblem/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,mark-rushakoff/influxdb