checker: fix embedded structure initialization warnings (fix #18376)

[yuyi98]

This PR fix embedded structure initialization warnings (fix #18376).

• Fix embedded structure initialization warnings.
• Add test.

struct Embedded {
	a &int
}

fn new_embedded() Embedded {
	return Embedded{
		a: unsafe { nil }
	}
}

struct Outer {
	Embedded
}

fn main() {
	o := Outer{
		Embedded: new_embedded()
	}
	println(o)
}

PS D:\Test\v\tt1> v run .
Outer{
    Embedded: Embedded{
        a: &nil
    }
}