fix: Check for empty messages before checking its properties in MuxRPCAPI

ssb/muxrpc.py

@@ -138,10 +138,11 @@ class MuxRPCAPI:
 
     async def __await__(self):
         async for req_message in self.connection:
-            body = req_message.body
             if req_message is None:
                 return
 
+            body = req_message.body
+
             if isinstance(body, dict) and body.get("name"):
                 self.process(self.connection, MuxRPCRequest.from_message(req_message))