Change get to return default value

Co-authored-by: Dumitru Gutu <dumitrugutu@users.noreply.github.com>

Author: J-Jaywalker

speechmatics/cli.py

@@ -590,7 +590,7 @@ def transcript_handler(message):
         transcripts.text += plaintext
 
     def get_channel(message):
-        return message.get("channel", None)
+        return message.get("channel")
 
     def audio_event_handler(message):
         if print_json: