fix(测试跟踪): 缺陷管理导入禅道缺陷报错

--bug=1019739 --user=宋昌昌 【测试跟踪】缺陷管理-导入-模版文件-导入失败 https://www.tapd.cn/55049933/s/1300238

test-track/backend/src/main/java/io/metersphere/service/issue/platform/AbstractIssuePlatform.java

@@ -384,6 +384,7 @@ public abstract class AbstractIssuePlatform implements IssuesPlatform {
 
     protected void addCustomFields(IssuesUpdateRequest issuesRequest, MultiValueMap<String, Object> paramMap) {
         List<CustomFieldItemDTO> customFields = issuesRequest.getRequestFields();
+        if (!CollectionUtils.isEmpty(customFields)) {
             customFields.forEach(item -> {
                 if (StringUtils.isNotBlank(item.getCustomData())) {
                     if (item.getValue() instanceof String) {
@@ -394,6 +395,7 @@ public abstract class AbstractIssuePlatform implements IssuesPlatform {
                 }
             });
         }
+    }
 
     protected Object getSyncJsonParamValue(Object value) {
         Map valObj = ((Map) value);