don't fetch alerts for empty item list

dist/datasource-zabbix/datasource.js

@@ -771,6 +771,9 @@ System.register(['lodash', 'app/core/utils/datemath', './utils', './migrations',
               var items = _.flatten(results);
               var itemids = _.map(items, 'itemid');
 
+              if (itemids.length === 0) {
+                return [];
+              }
               return _this11.zabbix.getAlerts(itemids);
             }).then(function (triggers) {
               triggers = _.filter(triggers, function (trigger) {

dist/test/datasource-zabbix/datasource.js

@@ -680,6 +680,9 @@ var ZabbixAPIDatasource = function () {
         var items = _lodash2.default.flatten(results);
         var itemids = _lodash2.default.map(items, 'itemid');
 
+        if (itemids.length === 0) {
+          return [];
+        }
         return _this11.zabbix.getAlerts(itemids);
       }).then(function (triggers) {
         triggers = _lodash2.default.filter(triggers, function (trigger) {

src/datasource-zabbix/datasource.js

@@ -576,6 +576,9 @@ class ZabbixAPIDatasource {
       let items = _.flatten(results);
       let itemids = _.map(items, 'itemid');
 
+      if (itemids.length === 0) {
+        return [];
+      }
       return this.zabbix.getAlerts(itemids);
     })
     .then(triggers => {