How return null value to empty string field

I have such struct in Golang application:

type Employee struct {
    ID int `json:"employee_id"`
    Email *string `json:"employee_email"`
    LastName *string `json:"employee_last_name"`
    FirstName *string `json:"employee_first_name"`
    Sex *string `json:"employee_sex"`
}

Some string fields of this struct can be empty. If I use *string application return me "". If use sql.NullString it return me for example such result:

"employee_last_name": {
    String: "",
    Valid: true
}

I want to show null is string field is empty.

In documentation I found such code:

type NullString struct {
    String string
    Valid  bool
}

func (ns *NullString) Scan(value interface{}) error {
    if value == nil {
        ns.String, ns.Valid = "", false
        return nil
    }
    ns.Valid = true
    return convertAssign(&ns.String, value)
}

func (ns NullString) Value() (driver.Value, error) {
    if !ns.Valid {
        return nil, nil
    }
    return ns.String, nil
}

As I understand this code could help me to solve my problem, right?! What I need to import in my application to use convertAssign function?