Update cwt/cose_message.py

Co-authored-by: Ajitomi Daisuke <dajiaji@gmail.com>

Author: kentakayama

cwt/cose_message.py

@@ -319,7 +319,7 @@ def detach_payload(self: Self) -> Tuple[COSEMessage, bytes]:
             ValueError: The payload does not exist.
         """
 
-        if not isinstance(self._payload, bytes):
+        if self._msg[2] is None:
             raise ValueError("The payload does not exist.")
 
         payload = self._payload