a-select设置了allowClear后，点击清除按钮会把绑定的属性从对象中删除

[wangfpp]

• I have searched the issues of this repository and believe that this is not a duplicate.

Version

4.2.6

Environment

mac window都可以稳定复现

Reproduction link

Steps to reproduce

1. reactive生命formModel
2. 绑定formItem select
3. select allowClear
4. 使用clearIcon清空 select的值
   此时formMedel的key也被清空了

What is expected?

应该只清空value 不应该清空key

What is actually happening?

应该只清空value 不应该清空key

[caoxicheng]

有些人的业务里空字符,undefined,null之类的有特殊的定义不是很麻烦.不如直接删除;
自己用后置图标的方案实现一下.

[github-actions]

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