Revert "fixup! fields.get_attribute: return None for FK lookups"

This reverts commit e0a7de6.

Author: blueyed

rest_framework/fields.py

@@ -98,10 +98,10 @@ def get_attribute(instance, attrs):
                 instance = instance[attr]
             else:
                 instance = getattr(instance, attr)
+                if instance is None:
+                    return None
         except ObjectDoesNotExist:
             return None
-        if instance is None:
-            return None
         if is_simple_callable(instance):
             try:
                 instance = instance()