fix chat and thumbnail bug (#2803)

### What problem does this PR solve?

1. fix white screen issue when chat response
2. thumbnail bug when document not support

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

---------

Co-authored-by: chongchuanbing <chongchuanbing@gmail.com>

web/src/pages/chat/markdown-content/index.tsx

@@ -45,7 +45,7 @@ const MarkdownContent = ({
   }, [content, loading, t]);
 
   useEffect(() => {
-    setDocumentIds(reference?.doc_aggs.map((x) => x.doc_id) ?? []);
+    setDocumentIds(reference?.doc_aggs?.map((x) => x.doc_id) ?? []);
   }, [reference, setDocumentIds]);
 
   const handleDocumentButtonClick = useCallback(