TNL-3316: Error on Insights performance report

Preserve DRF v2 behavior of defaulting to None
for missing keys in the grading policy dictionary.

openedx/core/djangoapps/content/course_structures/api/v0/serializers.py

@@ -1,6 +1,8 @@
 """
 API Serializers
 """
+from collections import defaultdict
+
 from rest_framework import serializers
 
 
@@ -11,6 +13,19 @@ class GradingPolicySerializer(serializers.Serializer):
     dropped = serializers.IntegerField(source='drop_count')
     weight = serializers.FloatField()
 
+    def to_representation(self, obj):
+        """
+        Return a representation of the grading policy.
+        """
+        # Backwards compatibility with the behavior of DRF v2.
+        # When the grader dictionary was missing keys, DRF v2 would default to None;
+        # DRF v3 unhelpfully raises an exception.
+        return dict(
+            super(GradingPolicySerializer, self).to_representation(
+                defaultdict(lambda: None, obj)
+            )
+        )
+
 
 # pylint: disable=invalid-name
 class BlockSerializer(serializers.Serializer):