return empty array for no datapoints

pkg/tsdb/cloudwatch/cloudwatch.go

@@ -316,6 +316,7 @@ func parseResponse(resp *cloudwatch.GetMetricStatisticsOutput, query *CloudWatch
 	for _, s := range append(query.Statistics, query.ExtendedStatistics...) {
 		series := tsdb.TimeSeries{
 			Tags:   map[string]string{},
+			Points: make([]tsdb.TimePoint, 0),
 		}
 		for _, d := range query.Dimensions {
 			series.Tags[*d.Name] = *d.Value