fix AggregateByRange() on empty data

pkg/timeseries/timeseries.go

@@ -176,10 +176,12 @@ func AggregateByRange(series []*TimeSeriesData, aggFunc AggFunc) *TimeSeriesData
 
 	value := aggFunc(aggregatedSeries)
 	aggregatedSeriesData := NewTimeSeriesData()
+	if len(aggregatedSeries) > 0 {
 		aggregatedSeriesData.TS = []TimePoint{
 			{Time: aggregatedSeries[0].Time, Value: value},
 			{Time: aggregatedSeries[aggregatedSeries.Len()-1].Time, Value: value},
 		}
+	}
 
 	return aggregatedSeriesData
 }