fix streaming "list index out of range"

lightrag/llm/openai.py

@@ -200,6 +200,15 @@ async def openai_complete_if_cache(
         async def inner():
             try:
                 async for chunk in response:
+                    # Check if choices exists and is not empty
+                    if not hasattr(chunk, 'choices') or not chunk.choices:
+                        logger.warning(f"Received chunk without choices: {chunk}")
+                        continue
+                    
+                    # Check if delta exists and has content
+                    if not hasattr(chunk.choices[0], 'delta') or not hasattr(chunk.choices[0].delta, 'content'):
+                        logger.warning(f"Received chunk without delta content: {chunk.choices[0]}")
+                        continue                    
                     content = chunk.choices[0].delta.content
                     if content is None:
                         continue