Fix behavior of heka_alarming.py for None service_data

Currently if service data has a key with None value:
`{a: None}`, function `grains_for_mine` raises exception.
It's incorrect. For right approach it should only pass this pair.
diff --git a/_modules/heka_alarming.py b/_modules/heka_alarming.py
index 8f86efd..efbd091 100644
--- a/_modules/heka_alarming.py
+++ b/_modules/heka_alarming.py
@@ -83,6 +83,8 @@
     """
     filtered_grains = {}
     for service_name, service_data in grains.items():
+        if not service_data:
+            continue
         alarm = service_data.get('alarm')
         if alarm:
             filtered_grains[service_name] = {'alarm': alarm}