type Name struct {
	First string `json:"first,omitempty"`
	Last string `json:"last,omitempty"`
}

type Person struct {
	Name Name `json:"name,omitempty"`
	Age int `json:"age"`
}

p := Person{
	Age: 42,
}
b, _ := json.Marshal(p)
fmt.Println(string(b))

{"age":42}

{"name":{},"age":42}

type Empty interface {
	IsEmpty() bool
}

func (n Name) IsEmpty() bool {
	return n.First == "" && n.Last == ""
}

if emp, ok := v.Interface().(Empty); ok {
	return emp.IsEmpty()
}