fix: guard empty chunks in stream (list index out of range)

app.py

@@ -71,6 +71,9 @@ def respond_streaming(user_message: str, history: list):
             temperature=0.3,
             stream=True,
         ):
+            # Guard against empty chunks at end of stream
+            if not chunk.choices:
+                continue
             delta = chunk.choices[0].delta.content
             if delta:
                 partial += delta