Assigning null to JSON fields instead of empty strings

Since empty string is the zero/default value for Go string , I decided to define all such fields as interface{} instead. for example

type student struct {
    FirstName  interface{} `json:"first_name"`
    MiddleName interface{} `json:"middle_name"`
    LastName   interface{} `json:"last_name"`
}

The application I am sending my data expect a null instead of an empty string if value is not available for that specific field.

Is this the correct approach or can someone please point me to something better than this.