Fix(Redis): Convert None task_id to empty string during serialization

Resolves DataError in Redis Streams when task_id is None by ensuring
it's serialized as an empty string instead of None, which Redis does
not support. Applies to ScheduleMessageItem.to_dict method.

Author: glin1993@outlook.com

src/memos/mem_scheduler/schemas/message_schemas.py

@@ -84,7 +84,7 @@ def to_dict(self) -> dict:
             "content": self.content,
             "timestamp": self.timestamp.isoformat(),
             "user_name": self.user_name,
-            "task_id": self.task_id,
+            "task_id": self.task_id if self.task_id is not None else "",
         }
 
     @classmethod