Fix API schema for Answer (#4054)

Related:
-
DIAGNijmegen/rse-gcapi#201 (comment)

This allows the deduced spec/schema to resolve to set `nullable: True`,
and subsequent derived models in gcapi to allow for `None` values.

Author: chrisvanrun

app/grandchallenge/reader_studies/serializers.py

@@ -207,9 +207,14 @@ class AnswerSerializer(HyperlinkedModelSerializer):
         required=False,
     )
     answer_image = HyperlinkedRelatedField(
-        read_only=True, view_name="api:image-detail"
+        read_only=True,
+        view_name="api:image-detail",
+        allow_null=True,
+    )
+    total_edit_duration = DurationField(
+        read_only=True,
+        allow_null=True,
     )
-    total_edit_duration = DurationField(read_only=True)
     # At the moment only non-ground-truth answers are created over REST
     is_ground_truth = BooleanField(read_only=True)