checker: fix nested option struct with required attr init (fix #17513)

[yuyi98]

This PR fix nested option struct with required attr init (fix #17513).

• Fix nested option struct with required attr init.
• Add test.

struct AA {
	bb ?BB // defaults to none
}

struct BB {
	x string [required]
}

fn main() {
	aa := AA{}
	println(aa)
	assert aa.bb == none
}

PS D:\Test\v\tt1> v run .
AA{
    bb: Option(error: none)
}