fix(llm): 修复文档为空时的段落命中率判断逻辑

- 将判断条件从 CollectionUtils.isNotEmpty(rateVOS) 修改为 CollectionUtils.isEmpty(rateVOS)
- 修复了当 rateVOS为空时应该返回 null 的逻辑错误

yudao-module-llm/yudao-module-llm-biz/src/main/java/cn/iocoder/yudao/module/llm/service/conversation/ConversationServiceImpl.java

@@ -575,7 +575,7 @@ public class ConversationServiceImpl implements ConversationService {
 
             paragraphHitRateListVO.setDocumentName(document.getDocumentName());
             List<ParagraphHitRateVO> rateVOS = parseEmbeddingResponse(response, knowledgeBase, chatReqVO.getUuid());
-            if (CollectionUtils.isNotEmpty(rateVOS)){
+            if (CollectionUtils.isEmpty(rateVOS)){
                 return null;
             }
             paragraphHitRateListVO.setParagraphHitRate(rateVOS);