Fix assistant message display content fallback logic

- Handle undefined vs empty string cases
- Prevent COT content keep rendering before </think> tag recieved

lightrag_webui/src/components/retrieval/ChatMessage.tsx

@@ -53,7 +53,7 @@ export const ChatMessage = ({ message }: { message: MessageWithError }) => { //
   // For assistant messages, we prefer displayContent but fallback to content for backward compatibility
   const finalDisplayContent = message.role === 'user'
     ? message.content
-    : displayContent || message.content || ''
+    : (displayContent !== undefined ? displayContent : (message.content || ''))
 
   // Load KaTeX dynamically
   useEffect(() => {