fix(项目设置): 自定义代码片段执行报错

--bug=1018680 --user=李玉号
【项目设置】自定义代码片段-创建代码片段-从API定义导入-测试-执行结果报错
https://www.tapd.cn/55049933/s/1273843

framework/sdk-parent/frontend/src/components/environment/snippet/custom-function.js

@@ -281,6 +281,9 @@ function _beanshellTemplate(obj) {
   }
   try {
     requestBody = JSON.stringify(requestBody);
+    if (!requestBody || requestBody === "null") {
+      requestBody = "";
+    }
   } catch (e) {
     requestBody = "";
   }