omitempty returns nil for defined empty sections

[dbarrosop]

Describe the bug

When parsing a document into a struct using pointers and omitempty, empty sections are interpreted as nil same as if they weren't defined at all.

The main issue with this behavior is that the action Marshal(Unmarshal(doc)) mutates the document.

To Reproduce

Run the following example:

package main

import (
	"fmt"

	"github.com/pelletier/go-toml/v2"
)

type A struct {
	Nullable *struct {
		A string
	} `toml:"nullable,omitempty"`
}

type Doc struct {
	A *A `toml:"a,omitempty"`
}

func main() {
	doc1 := []byte(`[a]
[a.nullable]
`)
	doc2 := []byte(`[a]`)

	var d1 Doc
	if err := toml.Unmarshal(doc1, &d1); err != nil {
		panic(err)
	}
	fmt.Println(d1.A.Nullable)

	var d2 Doc
	if err := toml.Unmarshal(doc2, &d2); err != nil {
		panic(err)
	}
	fmt.Println(d2.A)
}

which prints:

<nil>
<nil>

Expected behavior

I'd expect it to return &Nullable{} in the first case as the section was defined.

Versions

• go-toml: 60e4af8
• go: 1.20
• operating system: Linux

Additional context