checker: fix cast from an empty struct to option (fix #25566)

[StunxFS]

$ cat x.v
struct Struct {}

fn main() {
        s := ?Struct(Struct{})
        a := s or { panic('none') }
        println(a)
        assert a == Struct{}
}
$ ./v run x.v
Struct{}

Fix #25566.

[spytheman]

Excellent work.
Thank you.