fix: oneof type only write/size once

[Hchenn]

What type of PR is this?

What this PR does / why we need it (en: English/zh: Chinese):

en:
zh: pb marshal 默认对于空值是不写入编码的，但是 union 类型存在特例：如果声明了 union 的具体类型默认值，必须要写入这个类型，对端 receive 才能得知 "one of which type"
当前的 fastwrite 没有考虑到这点，直接跳过了带空值的 union 类型，导致接收端无法得知具体是哪个类型

Which issue(s) this PR fixes:

[xiaost]

actually, I don't know what this change fixed, and it doesn't come with unittest as well.