Fix panel error when no items returned by templated variable query.
Fix panel error when no items returned by templated variable query.

xiaohuiz/grafana-zabbix,xiaohuiz/grafana-zabbix,alexanderzobnin/grafana-zabbix,alexanderzobnin/grafana-zabbix,alexanderzobnin/grafana-zabbix,alexanderzobnin/grafana-zabbix,alexanderzobnin/grafana-zabbix,alexanderzobnin/grafana-zabbix