fix: correctly initialize AudioStream._processor

otherwise it will raise an exception when FrameProcessor isn't active

```
Traceback (most recent call last):
  File "/app/venv/lib/python3.13/site-packages/livekit/rtc/audio_stream.py", line 276, in _run
    if self._processor is not None and self._processor.enabled:
       ^^^^^^^^^^^^^^^
AttributeError: 'AudioStream' object has no attribute '_processor'
```

Author: davidzhao

livekit-rtc/livekit/rtc/audio_stream.py

@@ -105,8 +105,9 @@ def __init__(
         self._ffi_queue = FfiClient.instance.queue.subscribe(self._loop)
         self._queue: RingQueue[AudioFrameEvent | None] = RingQueue(capacity)
 
-        self._audio_filter_module = None
-        self._audio_filter_options = None
+        self._audio_filter_module: str | None = None
+        self._audio_filter_options: dict[str, Any] | None = None
+        self._processor: FrameProcessor[AudioFrame] | None = None
         if isinstance(noise_cancellation, NoiseCancellationOptions):
             self._audio_filter_module = noise_cancellation.module_id
             self._audio_filter_options = noise_cancellation.options