[Bug] 请求mocker保存报错：“send mock data unhandled error”

### Search before asking

- [X] I have searched the existing [issues](https://github.com/arextest/arex-agent-java/issues) before asking.

### AREX Test Service

AREX Java Agent (arextest/arex-agent-java)

### Current Behavior

报错截图如下

日志信息
```log
2024-07-11 21:43:28:558 [arex-data-save-handler-4-thread-1] WARN io.arex.inst.runtime.log.LogManager - [[title=arex.saveDataLoop]]send mock data unhandled error: Cannot invoke "String.getBytes(java.nio.charset.Charset)" because "original" is null
2024-07-11 21:43:28:558 [arex-data-save-handler-4-thread-1] WARN io.arex.inst.runtime.log.LogManager - [[title=arex.saveDataLoop]]send mock data unhandled error: Cannot invoke "String.getBytes(java.nio.charset.Charset)" because "original" is null
```

问题排查：defaultSerializer 为空导致，debug跟踪代码发现 ServiceLoader 只加载了两个serializer，不包含 `io.arex.foundation.serializer.jackson.JacksonSerializer#JacksonSerializer`；后续在保存 mocker 时，由于 defaultSerializer 为空导致程序执行异常，后续录制的mocker无法保存
**截图中的 classLoader 是 appClassLoader，如果是 spring 程序的话，是 tomcat loader，不会有问题**

### Expected Behavior

一般的场景：tomcat loader 加载时，会包含 `io.arex.foundation.serializer.jackson.JacksonSerializer#JacksonSerializer`

### Steps To Reproduce

非 tomcate 程序：如下截图的的 xxl 实例程序（jdk17）：

复现：
1、构建 arex agent，用jdk17
2、指定 agent 挂载参数启动 `xxl-job-executor-sample-frameless`
3、下发一个调度请求到 `xxl-job-executor-sample-frameless` 程序（开始录制）
4、接着会看到报错日志：

### Anything else

_No response_

### Are you willing to submit a pull request to fix on your own?

- [ ] Yes I am willing to submit a pull request on my own!

Start by tracing the ServiceLoader setup for defaultSerializer and the loading of io.arex.foundation.serializer.jackson.JacksonSerializer in the mock-save path. Reproduce with the JDK 17 xxl-job-executor-sample-frameless program and the agent attached, then verify that a non-Tomcat application loads the serializer and saves recorded mock data without the unhandled error.

Written by the indexing model from the issue text.