１．json.Marshalでstructにomitemptyを指定して nill -> null を行う
(1) tructにomitemptyを指定
(2) nill,　””　,　false　を判定
(3) nill の場合、nill -> null を行う

２．結論
・omitemptyを指定しても、実行結果ではフィールドは省略されていない

２．code

package main

import (
    "encoding/json"
    "fmt"
)

type Employee_s struct {
    Name        interface{}  `json:"name,omitempty"`
    Id          *interface{} `json:"id,omitempty"`
}

func main() {

  var NULL interface{};
    employee := [6] Employee_s{}

    employee[0].Name = "Name_1"
    employee[1].Name = nil
    employee[2].Name = NULL
    employee[3].Name = ""
    employee[4].Name = ``
    employee[5].Name = false

    isNull_1 := interface{}(nil)
    isNull_2 := interface{}(2)
    isNull_3 := interface{}("")

    var isNull_4 interface{}
    var isNull_5 interface{}
    var isNull_6 interface{}

    isNull_4 = nil
    isNull_5 = false
    isNull_6 = true

    employee[0].Id = &isNull_1
    employee[1].Id = &isNull_2
    employee[2].Id = &isNull_3
    employee[3].Id = &isNull_4
    employee[4].Id = &isNull_5
    employee[5].Id = &isNull_6

    json_string, _ := json.Marshal(employee)

    fmt.Println(string(json_string))
}

・実行結果

# go run test.go
[{"Name":"Name_1","id":null},{"Name":null,"id":2},{"Name":null,"id":""},{"Name":"","id":null},{"Name":"","id":false},{"Name":false,"id":true}]
#