Map entries with nil values are not marshaled

[burgerdev]

Describe the bug

I'm trying to marshal/unmarshal the type map[string]any. Marshaling an instance like {"key": nil} results in an entirely empty doc, but I expected the presence of "key" to be reflected in the doc. In particular, the key is lost in a round-trip to toml and back.

To Reproduce

package test

import (
	"testing"

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

const key = "foo"

func TestEncode(t *testing.T) {
	oldMap := map[string]any{key: nil}
	doc, err := toml.Marshal(&oldMap)
	if err != nil {
		t.Fatal(err)
	}

	var newMap map[string]any
	if err := toml.Unmarshal(doc, &newMap); err != nil {
		t.Fatal(err)
	}

	if _, ok := newMap["foo"]; !ok {
		t.Fatalf("%v does not contain key %q", newMap, key)
	}
}

This fails with

=== RUN   TestEncode
    kata_runtime_test.go:27: map[] does not contain key "foo"
--- FAIL: TestEncode (0.00s)

Expected behavior

I expected the Marshal function to produce an empty table at foo:

[foo]

Versions

• go-toml: v2.2.3
• go: go version go1.23.3 linux/amd64
• operating system: Linux