msgproto: Fix return type for create_command()

Return an empty list instead of an emptry string if no command found.
This improves compatibility within console.py on python3.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/msgproto.py

@@ -324,7 +324,7 @@ class MessageParser:
     def create_command(self, msg):
         parts = msg.strip().split()
         if not parts:
-            return ""
+            return []
         msgname = parts[0]
         mp = self.messages_by_name.get(msgname)
         if mp is None: