@RequestBody 请求体为null，不能拦截，会将空对象传导到controller

接口已经定义body体不能为空，但body请求体 什么也不传，参数校验拦截不了。
parameters:
- in: "body"
name: "xxx "
required: true
servicecomb版本 2.7.4
期待报4XX错误 。

Start by reproducing the reported behavior on ServiceComb Java Chassis 2.7.4 with an endpoint whose @RequestBody is required but receives no body. Trace why validation does not intercept the null request and verify that the completed behavior returns a 4XX response instead of passing an empty object to the controller.

Written by the indexing model from the issue text.