fix(测试跟踪): 脑图创建用例报错

--bug=1015526 --user=陈建星 [测试跟踪]github#16457升级到v2.0.0版本后，脑图新增用例保存报错500，后台服务日志报NullPointerException {#_orginal_url#}

backend/src/main/java/io/metersphere/track/service/TestCaseService.java

@@ -238,8 +238,10 @@ public class TestCaseService {
         testCaseMapper.insert(request);
         saveFollows(request.getId(), request.getFollows());
         List<CustomFieldResource> addFields = request.getAddFields();
+        if (CollectionUtils.isNotEmpty(addFields)) {
             addFields.addAll(request.getEditFields());
             customFieldTestCaseService.addFields(request.getId(), addFields);
+        }
         return request;
     }