fix(测试跟踪): 功能用例的自定义字段导出失败

--bug=1015705 --user=陈建星 【测试跟踪】github #16739，用例的自定义字段不能导出 https://www.tapd.cn/55049933/s/1221273

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

@@ -1667,16 +1667,18 @@ public class TestCaseService {
                     CustomFieldResource field = fields.get(index);
                     //进行key value对换
                     String id = field.getFieldId();
+                    if (StringUtils.isNotBlank(field.getValue())) {
                         String value = JSONObject.parse(field.getValue()).toString();
-                    if (customSelectValueMap.containsKey(id)) {
-                        if (customSelectValueMap.get(id).containsKey(value)) {
+                        if (customSelectValueMap.containsKey(id)
+                            && customSelectValueMap.get(id).containsKey(value)) {
                             value = customSelectValueMap.get(id).get(value);
                         }
-                    }
                         map.put(customNameMap.get(id), value);
                     }
+                }
                 data.setCustomDatas(map);
             } catch (Exception e) {
+                LogUtil.error(e);
             }
             list.add(data);
         });