Marshalled JSON includes attributes with `Present: false`

[skplunkerin]

In the provided code, when unmarshalling the JSON the Weight attribute is correctly set to present: false, valid: false as "Weight" isn't specified in the JSON.

However, when marshalling the struct back to JSON the output includes the Weight attribute with a value of null. This is unexpected as I would assume Present: false would omit the attribute from the marshalled JSON (and that the marshalled JSON should match the original JSON).

Steps to Reproduce

1. Run the following code: (modified from this example in the project README.md)

package main

import (
	"encoding/json"
	"fmt"

	"github.com/LukaGiorgadze/gonull"
)

type MyCustomInt int
type MyCustomFloat32 float32

type Person struct {
	Name    string
	Age     gonull.Nullable[MyCustomInt]     // present: true,  valid: true
	Address gonull.Nullable[string]          // present: true,  valid: false
	Height  gonull.Nullable[MyCustomFloat32] // present: true,  valid: false
	Weight  gonull.Nullable[string]          // present: false, valid: false
}

func main() {
	jsonData := []byte(`{"Name":"Alice","Age":15,"Address":null,"Height":null}`)

	var person Person
	err := json.Unmarshal(jsonData, &person)
	if err != nil {
		panic(err)
	}
	fmt.Printf("Unmarshalled Person: %+v\n", person)

	marshalledData, err := json.Marshal(person)
	if err != nil {
		panic(err)
	}
	fmt.Printf("Marshalled JSON: %s\n", string(marshalledData))
}

Expected Behavior

The marshalled JSON should not include the Weight attribute as it was not present in the original JSON (Present: false):

{
  "Name": "Alice",
  "Age": 15,
  "Address": null,
  "Height": null
}

Actual Behavior

The marshalled JSON includes the Weight attribute with a value of null:

{
  "Name": "Alice",
  "Age": 15,
  "Address": null,
  "Height": null,
  "Weight": null // <-- not expected, this should not be included
}

Environment

• Go version: 1.22
• gonull version: 1.2.0
• OS: The Go Playground

Additional Context

This issue was discovered while testing the unmarshalling and marshalling of JSON data with optional fields in the Go Playground. The expected behavior is based on the assumption that Present: false should result in the attribute being omitted from the marshalled JSON.