Fix yamllint errors

prometheus/defaults/main.yml

@@ -2,5 +2,8 @@
 prometheus_args: ""
 prometheus:
   rules:
-    alert:
-    recording:
+    # These variables intentionally left null.  They need to exist, so that the
+    # template works, but they need to have a value (empty string does not
+    # work).
+    alert: null
+    recording: null