fix last 3 yaml lint failures

- add missing --- to notification-policy.yml
- prometheus.yml: replace commented-out template defaults with empty lists

ansible/services/grafana/provisioning/alerting/notification-policy.yml

@@ -1,3 +1,4 @@
+---
 apiVersion: 1
 
 # Notification routing policy.

ansible/services/prometheus/prometheus.yml

@@ -11,12 +11,9 @@ global:
 alerting:
   alertmanagers:
     - static_configs:
-        - targets:
-            # - alertmanager:9093
+        - targets: []
 
-rule_files:
-    # - "first_rules.yml"
-    # - "second_rules.yml"
+rule_files: []
 
 scrape_configs:
   - job_name: "prometheus"