Golang: Validate Struct field of type string to be one of specific values

golang version: 1.18.3

validator: github.com/go-playground/validator/v10

I want to validate an incoming JSON payload after loaded into nested struct data structure. Here's my incoming JSON payload,

{
  "name": "Duro",
  "gender": "MALE",
  "tier": 3,
  "mobileNumber": "0356874586",
  "address": {
    "city": "Tokyo",
    "street": "Shibaura St"
  },
  "children":[
    {
      "title": "Mr",
      "lastName": "Takayashi"
    }
  ],
  "isEmployed": false,
  "requestedAt": "2022-01-10T03:30:12.639Z"
}

Here's my user.go file,

package main

type User struct {
        Name                   string                     `validate:"required"`
        Gender                 string                     `validate:"required,oneof=MALE FEMALE"`
        Tier                   *uint8                     `validate:"required,eq=0|eq=1|eq=2|eq=3"`
        MobileNumber           string                     `validate:"required"`
        Email                  string
        Address                *Address  `validate:"required"`
        Children               []Child   `validate:"required,dive"`
        IsEmployed             *bool     `validate:"required"`
        PreferredContactMethod string    `validate:"oneof=EMAIL PHONE POST SMS"`
        RequestedAt            time.Time `validate:"required"`
    }
    type Address struct {
        City   string `validate:"required"`
        Street string `validate:"required"`
    }
    
    type Child struct {
        Title     string `validate:"required"`
        FirstName string
        LastName  string `validate:"required"`
    }

Here's my test function

func TestUserPayload(t *testing.T) {
    validate := validator.New()
    var u User

    err := json.Unmarshal([]byte(jsonData), &u)
    if err != nil {
        panic(err)
    }

    err := validate.Struct(&u)
    if err != nil {
        t.Errorf("error %v", err)
    }

}

This test fails with error,

error Key: 'User.PreferredContactMethod' Error:Field validation for 'PreferredContactMethod' failed on the 'oneof' tag

This happens because Go assigns empty string to User.PreferredContactMethod struct field. Since PreferredContactMethod is NOT a required field, I DON'T want to see this validation error when json payload doesn't have it. How can I avoid this error message when the json payload doesn't have preferredContactMethod field?

If you have better alternatives to achieve this validation, happy to hear them as well.

Here's the code in Go Playground