Support None if initial part of dotted lookup returns None. Closes #1223.

rest_framework/fields.py

@@ -57,6 +57,9 @@ def get_attribute(instance, attrs):
     Also accepts either attribute lookup on objects or dictionary lookups.
     """
     for attr in attrs:
+        if instance is None:
+            # Break out early if we get `None` at any point in a nested lookup.
+            return None
         try:
             instance = getattr(instance, attr)
         except ObjectDoesNotExist: