Problems: fix empty problems list when null value used as a filter

src/datasource-zabbix/datasource.ts

@@ -768,7 +768,7 @@ function zabbixItemIdsTemplateFormat(value) {
  */
 function replaceTemplateVars(templateSrv, target, scopedVars) {
   let replacedTarget = templateSrv.replace(target, scopedVars, zabbixTemplateFormat);
-  if (target !== replacedTarget && !utils.isRegex(replacedTarget)) {
+  if (target && target !== replacedTarget && !utils.isRegex(replacedTarget)) {
     replacedTarget = '/^' + replacedTarget + '$/';
   }
   return replacedTarget;

src/datasource-zabbix/partials/query.editor.html

@@ -160,8 +160,7 @@
         ng-class="{
           'zbx-variable': ctrl.isVariable(ctrl.target.trigger.filter),
           'zbx-regex': ctrl.isRegex(ctrl.target.trigger.filter)
-        }"
-        empty-to-null>
+        }">
     </div>
 
     <div class="gf-form max-width-20" ng-show="ctrl.target.queryType == editorMode.PROBLEMS">