Refuse an empty file before the GPU is reserved

An audio file of no frames passed the checks in process_fn, since soundfile reads it without complaint and the limits checked are maximums, so it reached the GPU call and failed there, spending the caller's ZeroGPU allowance. It is refused with a message first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app.py

@@ -144,6 +144,8 @@ def process_fn(
         duration = sf.info(input_audio_path).duration
     except Exception as exc:
         raise gr.Error(f"Could not read the audio file: {exc}") from exc
+    if duration == 0:
+        raise gr.Error("The audio file is empty.")
     if gpu_seconds(input_audio_path, variant, instruments_text, use_sampling,
                    temperature) > MAX_GPU_SECONDS:
         raise gr.Error(