Fix One field that is required=False, allow_null=True and not provided. to_representation should not explicitly set it to None

rest_framework/fields.py

@@ -450,10 +450,10 @@ class Field:
         except (KeyError, AttributeError) as exc:
             if self.default is not empty:
                 return self.get_default()
-            if self.allow_null:
-                return None
             if not self.required:
                 raise SkipField()
+            if self.allow_null:
+                return None
             msg = (
                 'Got {exc_type} when attempting to get a value for field '
                 '`{field}` on serializer `{serializer}`.\nThe serializer '