Bring check for null fk to `BaseSerializer.to_representation`

rest_framework/fields.py

@@ -778,8 +778,6 @@ class UUIDField(Field):
         return data
 
     def to_representation(self, value):
-        if value is None:
-            return None
         if self.uuid_format == 'hex_verbose':
             return str(value)
         else:

rest_framework/serializers.py

@@ -464,9 +464,13 @@ class Serializer(BaseSerializer):
             except SkipField:
                 continue
 
-            if attribute is None:
-                # We skip `to_representation` for `None` values so that
-                # fields do not have to explicitly deal with that case.
+            # We skip `to_representation` for `None` values so that fields do
+            # not have to explicitly deal with that case.
+            #
+            # For related fields with `use_pk_only_optimization` we need to
+            # resolve the pk value.
+            check_for_none = attribute.pk if isinstance(attribute, PKOnlyObject) else attribute
+            if check_for_none is None:
                 ret[field.field_name] = None
             else:
                 ret[field.field_name] = field.to_representation(attribute)