Fix problems panel broken when selecting triggers (#1960)

This PR fixes an issue where the problems panel breaks when you change
to query type to `Triggers`.

![](https://grafana.zendesk.com/attachments/token/MNBgVTPRKZkwa6TsMBbtBQdIp/?name=image.png)

How to reproduce:

1. Go to `Zabbix data source features` dashboard
2. Open Problems panel
3. Change`Query type` to `Triggers`
4. Select `Problems` from `Count by`
5. Change the `Group` to `Backend`
6. The problems panel should not error

---------

Co-authored-by: Zoltán Bedi <zoltan.bedi@gmail.com>

.changeset/purple-carrots-rhyme.md

@@ -0,0 +1,5 @@
+---
+'grafana-zabbix': patch
+---
+
+Fix problems panel broken when selecting triggers