groupBy(): minor refactor

dist/test/datasource-zabbix/timeseries.js

@@ -108,6 +108,10 @@ function groupBy(datapoints, interval, groupByCallback) {
 }
 
 function groupBy_perf(datapoints, interval, groupByCallback) {
+  if (datapoints.length === 0) {
+    return [];
+  }
+
   var ms_interval = utils.parseInterval(interval);
   var grouped_series = [];
   var frame_values = [];