fix:修复可能报空的风险。

mooctest-site-server/src/main/java/cn/iselab/mooctest/site/service/common/impl/PdfServiceImpl.java

@@ -173,7 +173,7 @@ public class PdfServiceImpl extends BaseService implements PdfService {
 
 
             //每一个case的提交列表
-            if (!uploadRecords.isEmpty()){
+            if (uploadRecords!=null&&!uploadRecords.isEmpty()){
                 pdf.setUploadRecordList(uploadRecords);
             }else {
                 pdf.setUploadRecordList(new ArrayList<>());