Set grading policy to an empty dict if it can't be decoded from json

common/lib/xmodule/xmodule/course_module.py

@@ -246,7 +246,7 @@ class CourseDescriptor(SequenceDescriptor):
             policy = json.loads(cls.read_grading_policy(paths, system))
         except ValueError:
             system.error_tracker("Unable to decode grading policy as json")
-            policy = None
+            policy = {}
 
         # cdodge: import the grading policy information that is on disk and put into the
         # descriptor 'definition' bucket as a dictionary so that it is persisted in the DB