fixed signature

taipy/core/submission/_submission_manager.py

@@ -35,7 +35,7 @@ class _SubmissionManager(_Manager[Submission], _VersionMixin):
         return cls._repository._load_all(filters)
 
     @classmethod
-    def _create(cls, entity_id: str, entity_type: str, entity_config: str) -> Submission:
+    def _create(cls, entity_id: str, entity_type: str, entity_config: Optional[str]) -> Submission:
         submission = Submission(entity_id=entity_id, entity_type=entity_type, entity_config_id=entity_config)
         cls._set(submission)