fix: achieved goal in bdi core

ref: N25B-400

src/control_backend/schemas/belief_message.py

@@ -11,7 +11,7 @@ class Belief(BaseModel):
     """
 
     name: str
-    arguments: list[str] | None
+    arguments: list[str] | None = None
 
     # To make it hashable
     model_config = {"frozen": True}