fix meta/salt with missing pillar

Change-Id: I43333c76bb3814d2c6b98849df109ef97523dc7e
diff --git a/prometheus/meta/salt.yml b/prometheus/meta/salt.yml
index 9343f83..a886376 100644
--- a/prometheus/meta/salt.yml
+++ b/prometheus/meta/salt.yml
@@ -1,5 +1,5 @@
 grain:
-  {%- if pillar.prometheus is defined and pillar.prometheus.collector is defined %}
+  {%- if pillar.get('prometheus', {}).get('collector') %}
     {%- from "prometheus/map.jinja" import server as prometheus_server with context %}
   prometheus:
     {%- set service_grains = {'prometheus': {'server': {'alert': {}, 'recording': {}, 'target': {}}}} %}