Incorrect nil copy

https://go.dev/play/p/DwZgPX8HhBs

```go
package main

import (
	"fmt"

	"golang.design/x/reflect"
)

type ListNode struct {
	Val  int
	Next *ListNode
}

func main() {
	n := ListNode{Val: 1}
	n2 := reflect.DeepCopy(n)
	fmt.Printf("%+v %+v\n", n, n2)
	fmt.Println(n2.Next.Val) // 0
}
```

`n.Next` is nil, but becomes `&ListNode{}` after the deep copy.