Update app.py

fixing runtime error

app.py

@@ -7,7 +7,7 @@ model = whisper.load_model("base")
 def transcribe_audio(audio_file):
     # Check file size (e.g., 25MB limit)
     if os.path.getsize(audio_file.name) > 25 * 1024 * 1024:
-    return "Error: File size exceeds 25MB limit.", ""
+        return "Error: File size exceeds 25MB limit.", None
 
     result = model.transcribe(audio_file.name)
     output_filename = os.path.splitext(os.path.basename(audio_file.name))[0] + ".txt"