fix: prevent save_docs_to_vector_db crash on empty result.ids (#20275)

Add check that result.ids exists and has length > 0 before accessing result.ids[0]. Prevents IndexError when query returns empty results.

backend/open_webui/routers/retrieval.py

@@ -1308,7 +1308,7 @@ def save_docs_to_vector_db(
             filter={"hash": metadata["hash"]},
         )
 
-        if result is not None:
+        if result is not None and result.ids and len(result.ids) > 0:
             existing_doc_ids = result.ids[0]
             if existing_doc_ids:
                 log.info(f"Document with hash {metadata['hash']} already exists")