fix: reduce debug log message amount when no audio received

Used to be every `timeout` time that we'd get the message that no audio data is received. Now only the first time since no data is received.

ref: N25B-213

src/control_backend/agents/vad_agent.py

@@ -58,8 +58,10 @@ class Streaming(CyclicBehaviour):
     async def run(self) -> None:
         data = await self.audio_in_poller.poll()
         if data is None:
-            logger.debug("No audio data received. Discarding buffer until new data arrives.")
-            self.audio_buffer = np.array([], dtype=np.float32)
+            if len(self.audio_buffer) > 0:
+                logger.debug("No audio data received. Discarding buffer until new data arrives.")
+                self.audio_buffer = np.array([], dtype=np.float32)
+                self.i_since_speech = 100
             return
 
         # copy otherwise Torch will be sad that it's immutable