Alerting: Don't when no scraping error ever happened.

This makes the alert I defined in !188 (merged) not go off if there aren't any scraping errors in the result set.

nixos/modules/monitoring/server/grafana-dashboards/meta-monitoring.json

@@ -52,7 +52,7 @@
         "frequency": "1m",
         "handler": 1,
         "name": "Scraping down",
-        "noDataState": "no_data",
+        "noDataState": "ok",
         "notifications": []
       },
       "aliasColors": {},
@@ -86,7 +86,7 @@
       },
       "lines": false,
       "linewidth": 1,
-      "nullPointMode": "null",
+      "nullPointMode": "null as zero",
       "options": {
         "alertThreshold": false
       },