groupBy(): minor refactor

dist/datasource-zabbix/timeseries.js

@@ -83,6 +83,10 @@ System.register(['lodash', './utils'], function (_export, _context) {
   }
 
   function groupBy_perf(datapoints, interval, groupByCallback) {
+    if (datapoints.length === 0) {
+      return [];
+    }
+
     var ms_interval = utils.parseInterval(interval);
     var grouped_series = [];
     var frame_values = [];