fix(测试跟踪): 修复功能用例没有预期步骤时导出excel后相关字段变为null的问题

--bug=1012886
--user=宋天阳
【测试跟踪】github#13201
xmind上传没有预期结果的步骤，导出excel版本用例，该步骤显示成null
https://www.tapd.cn/55049933/s/1156560

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

@@ -1485,7 +1485,6 @@ public class TestCaseService {
             data.setPrerequisite(t.getPrerequisite());
             data.setTags(t.getTags());
             if (StringUtils.equals(t.getMethod(), "manual") || StringUtils.isBlank(t.getMethod())) {
-
                 if (StringUtils.equals(data.getStepModel(), TestCaseConstants.StepModel.TEXT.name())) {
                     data.setStepDesc(t.getStepDescription());
                     data.setStepResult(t.getExpectedResult());
@@ -1508,8 +1507,16 @@ public class TestCaseService {
                     if (CollectionUtils.isNotEmpty(jsonArray)) {
                         for (int j = 0; j < jsonArray.size(); j++) {
                             int num = j + 1;
-                            step.append(num + "." + jsonArray.getJSONObject(j).getString("desc") + "\n");
-                            result.append(num + "." + jsonArray.getJSONObject(j).getString("result") + "\n");
+                            String stepItem = jsonArray.getJSONObject(j).getString("desc");
+                            if(StringUtils.isEmpty(stepItem)){
+                                stepItem = "";
+                            }
+                            step.append(num + "." + stepItem + "\n");
+                            String resultItem = jsonArray.getJSONObject(j).getString("result");
+                            if(StringUtils.isEmpty(resultItem)){
+                                resultItem = "";
+                            }
+                            result.append(num + "." + resultItem + "\n");
 
                         }
                     }