Serialize template rate laws to be None if the rate law doesn't exist

mira/metamodel/templates.py

@@ -458,7 +458,7 @@ def from_json(cls, data, rate_symbols=None) -> "Template":
 
     @field_serializer('rate_law')
     def serialize_expression(self, rate_law):
-        return str(rate_law)
+        return str(rate_law) if rate_law is not None else None
 
     def is_equal_to(self, other: "Template", with_context: bool = False,
                     config: Config = None) -> bool:

tests/test_model_api.py

@@ -589,18 +589,19 @@ def test_n_way_comparison_askenet(self):
             "exclude_defaults": True,
             "exclude_unset": True,
             "exclude_none": True,
-            # This key is outdated in Pydantic2
-            # "skip_defaults": True,
         }
         # Compare the ModelComparisonResponse models
         # If assertion fails the diff is printed
+        # Use model dump as Pydantic objects have an attribute
+        # "model_fields_set" that contain explicitly set attributes even if
+        # are None
         assert local_response.model_dump() == resp_model.model_dump()
         local_sorted_str = sorted_json_str(
-            json.loads(local_response.model_dump_json()),
+            json.loads(local_response.model_dump_json(**dict_options)),
             skip_empty=True
         )
         resp_sorted_str = sorted_json_str(
-            json.loads(resp_model.model_dump_json()),
+            json.loads(resp_model.model_dump_json(**dict_options)),
             skip_empty=True
         )
         self.assertEqual(local_sorted_str, resp_sorted_str)