datasource: fix disable query when using mixed datasource (#16409)
datasource: fix disable query when using mixed datasource (#16409)

This fixes an error that was thrown when one or several queries
was disabled using the mixed datasource.

Fixes #12977
grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana