We need empty {} to evaluate alerts

For some alerts we use reclass to get variables
(ex. threshold). To get those reclass variables
we use `prometheus_server` dict, which on nodes
without prometheus class is None.
We need to provide at least empty dict in that
case.

Change-Id: Ic1f2c1776c4852701dcf9f8a76f933926b2d9aec

prometheus/map.jinja

@@ -1,5 +1,9 @@
 {% set server = salt['grains.filter_by']({
+  'default': {
+  },
 }, merge=salt['pillar.get']('prometheus:server')) %}
 
 {% set alertmanager = salt['grains.filter_by']({
+  'default': {
+  },
 }, merge=salt['pillar.get']('prometheus:alertmanager')) %}