Marshal ignores omitempty struct tag on custom type

[IncSW]

go: go1.16 windows/amd64
go-json: master (v0.4.12-0.20210411072911-c74c2078ead8)

package test

import (
	"encoding/json"
	"strconv"
	"testing"

	gojson "github.com/goccy/go-json"
)

type CustomInt int

func (i CustomInt) MarshalJSON() ([]byte, error) {
	return []byte(strconv.Itoa(int(i))), nil
}

type Foo struct {
	Bar CustomInt `json:"b,omitempty"`
}

func TestOmitempty(t *testing.T) {
	foo := &Foo{}

	data, _ := json.Marshal(foo)
	t.Log(string(data)) // {}

	data, _ = gojson.Marshal(foo)
	t.Log(string(data)) // {"b":0}
}