ci: Only set GRAFANA_TOKEN when necessary
The CI is always setting GRAFANA_TOKEN to $GRAFANA9_TOKEN value to make it possible to communicate with two grafana instances, where the dashboard exporter tool only use the newer one.
But when GRAFANA9_TOKEN is missing, the GRAFANA_TOKEN variable is set to
a empty string, and the grafana-client
library thinks it is a valid
token, resulting in an unauthorized access.