set characteristic_group_id to None when empty see HEA-872

Author: girumb

pipelines/jobs/metadata.py

@@ -73,7 +73,7 @@ def load_metadata_for_model(context: OpExecutionContext, sheet_name: str, model:
     if "activity_type" in df:
         df["activity_type"] = df["activity_type"].replace("", ActivityLabel.LivelihoodActivityType.LIVELIHOOD_ACTIVITY)
     if "characteristic_group_id" in df:
-        df["characteristic_group_id"] = df["characteristic_group_id"].replace("", None)
+        df["characteristic_group_id"] = df["characteristic_group_id"].astype(object).replace("", None)
 
     if model_name == "ClassifiedProduct":
         existing_instances = {instance.pk: instance for instance in model.objects.filter(pk__in=df["cpc"])}