This PR fixes an issue where the problems panel breaks when you change
to query type to `Triggers`.
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
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Co-authored-by: Zoltán Bedi <zoltan.bedi@gmail.com>
