Fix grains generation for broken formulas

Avoid failing if some formula has enabled support for sensu but doesn't
provide valid data in meta/sensu.yml

Change-Id: I4b6bb0371c55f7fee091554dc879dcab54895de7
diff --git a/sensu/meta/salt.yml b/sensu/meta/salt.yml
index dac6991..d30d6aa 100644
--- a/sensu/meta/salt.yml
+++ b/sensu/meta/salt.yml
@@ -18,12 +18,14 @@
   {%- set grains_fragment_file = service_name+'/meta/sensu.yml' %}
   {%- macro load_grains_file() %}{% include grains_fragment_file %}{% endmacro %}
   {%- set grains_yaml = load_grains_file()|load_yaml %}
-  {%- for check_name, check in client.get('check', {}).items() %}
-    {%- if check_name in grains_yaml.check and not check.get('enabled', True) %}
-      {%- do grains_yaml.check.pop(check_name) %}
-    {%- endif %}
-  {%- endfor %}
-  {%- do service_grains.sensu.check.update(grains_yaml.check) %}
+  {%- if grains_yaml %}
+    {%- for check_name, check in client.get('check', {}).items() %}
+      {%- if check_name in grains_yaml.check and not check.get('enabled', True) %}
+        {%- do grains_yaml.check.pop(check_name) %}
+      {%- endif %}
+    {%- endfor %}
+    {%- do service_grains.sensu.check.update(grains_yaml.check) %}
+  {%- endif %}
   {%- endif %}
   {%- endfor %}
   sensu: