Fix NoneType crash in _process_single when VAD filters all audio

When VAD removes all speech from an audio chunk, transcriber.transcribe() returns (None, info). Calling list(None) raises TypeError. The _process_multi path already handles this case; this aligns _process_single to match.

whisper_live/batch_inference.py

@@ -213,7 +213,7 @@ class BatchInferenceWorker:
                 vad_parameters=req.vad_parameters if req.use_vad else None,
             )
             # Materialize the generator into a list
-            req.result = list(result)
+            req.result = list(result) if result is not None else []
             req.info = info
         except Exception as e:
             req.error = e