Import default grading_policy was setting it to a unicode empty string

rather than an empty dict and thus screwing up later updates.

common/lib/xmodule/xmodule/course_module.py

@@ -163,8 +163,8 @@ class CourseDescriptor(SequenceDescriptor):
     @classmethod
     def read_grading_policy(cls, paths, system):
         """Load a grading policy from the specified paths, in order, if it exists."""
-        # Default to a blank policy
-        policy_str = '""'
+        # Default to a blank policy dict
+        policy_str = '{}'
 
         for policy_path in paths:
             if not system.resources_fs.exists(policy_path):