fix(接口测试): 修复json-schema预览报错空指针的缺陷

--bug=1033026 --user=王孝刚
【接口测试】接口定义-编辑接口的请求体的json-schema格式，导入后模版和预览显示还是原始的
https://www.tapd.cn/55049933/s/1437834

api-test/backend/src/main/java/io/metersphere/api/exec/generator/JSONSchemaBuilder.java

@@ -116,7 +116,7 @@ public class JSONSchemaBuilder {
             propertyObjType = object.get(PropertyConstant.TYPE).getAsString();
         }
         if (propertyObjType.equals(PropertyConstant.STRING) || propertyObjType.equals(PropertyConstant.ENUM)) {
-            concept.put(propertyName, FormatterUtil.getMockValue(object));
+            concept.put(propertyName, object.has(PropertyConstant.MOCK) ? FormatterUtil.getMockValue(object) : StringUtils.EMPTY);
         } else if (propertyObjType.equals(PropertyConstant.INTEGER) || propertyObjType.equals(PropertyConstant.NUMBER)) {
             try {
                 concept.put(propertyName, 0);