Fix: ensure top_k defaults in _faiss_search to prevent NoneType error

chatbot_retriever.py

@@ -364,6 +364,7 @@ def _ensure_index_built():
 
 
 def _faiss_search(query: str, top_k: int = TOP_K_DOCS, subject: Optional[str] = None):
+    top_k = top_k or TOP_K_DOCS
     faiss_data = hybrid_retrieve._faiss
     if not faiss_data:
         return []