app.py: return nothing if audio is not provided

app.py

@@ -130,7 +130,7 @@ def main():
             if free_tier and past_requests_ >= FREE_TIER:
                 gr.Warning("Free tier exceeded, please reach out to get your API key")
                 logging.info(f"Free tier exceeded for {ip_}")
-                return None
+                return
             logging.info(f"Total number of requests: {counter}. {past_requests_} for {ip_}")
             counter += 1
             users[ip_] = past_requests_ + 1