add optional to channel_diarization_labels

Author: James Walker

speechmatics/models.py

@@ -134,7 +134,7 @@ def asdict(self) -> Dict[Any, Any]:
     transcript_filtering_config: Optional[dict] = None
     """Configuration for applying filtering to the transcription."""
 
-    channel_diarization_labels: List[str] = None
+    channel_diarization_labels: Optional[List[str]] = None
     """Add your own speaker or channel labels to the transcript"""