refactor: [conversable_agent] remove raise from property access (microsoft#2006)

* refactor: [conversable_agent] remove raise from property access

raising exception seems drastic, can we return None and let the called fail in case it receives a None?

* Update conversable_agent.py

---------

Co-authored-by: Chi Wang <[email protected]>

Author: lalo

autogen/agentchat/conversable_agent.py

@@ -270,13 +270,10 @@ def description(self, description: str):
         self._description = description
 
     @property
-    def code_executor(self) -> CodeExecutor:
-        """The code executor used by this agent. Raise if code execution is disabled."""
+    def code_executor(self) -> Optional[CodeExecutor]:
+        """The code executor used by this agent. Returns None if code execution is disabled."""
         if not hasattr(self, "_code_executor"):
-            raise ValueError(
-                "No code executor as code execution is disabled. "
-                "To enable code execution, set code_execution_config."
-            )
+            return None
         return self._code_executor
 
     def register_reply(