[client] Fix export an entity even if the status worfklow does not exist (#636)

Author: helene-nguyen

pycti/utils/opencti_stix2.py

@@ -1515,7 +1515,7 @@ def generate_export(self, entity: Dict, no_custom_attributes: bool = False) -> D
         if "tasks" in entity:
             del entity["tasks"]
 
-        if "status" in entity:
+        if "status" in entity and entity["status"] is not None:
             entity["x_opencti_workflow_id"] = entity["status"].get("id")
         if "status" in entity:
             del entity["status"]