String method call panics on empty document node

[bcho]

Describe the bug

The ast.DocumentNode panics on empty document, example program:

https://go.dev/play/p/YmrzDKhtFNp

package main

import (
	"fmt"

	"github.com/goccy/go-yaml/lexer"
	"github.com/goccy/go-yaml/parser"
)

func main() {
	f, err := parser.Parse(lexer.Tokenize(""), 0)
	if err != nil {
		panic(err)
	}
	fmt.Println(f.String())
}

The above program fails with panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x4ada54]

goroutine 1 [running]:
github.com/goccy/go-yaml/ast.(*DocumentNode).String(0xc0000162a0)
	/tmp/gopath1302303734/pkg/mod/github.com/goccy/go-yaml@v1.15.5/ast/ast.go:578 +0xb4
github.com/goccy/go-yaml/ast.(*File).String(0x0?)
	/tmp/gopath1302303734/pkg/mod/github.com/goccy/go-yaml@v1.15.5/ast/ast.go:535 +0x78
main.main()
	/tmp/sandbox432899830/prog.go:15 +0x34

This seems to be an regression introduced in v1.15.0 as the above same program works in v1.14.3 :

https://go.dev/play/p/VubIuMgDfYQ

I believe this is because now the body of the document node can be nil due to the new implementation in here:

go-yaml/parser/parser.go

Lines 107 to 109 in 3399084

	if len(docGroup.Tokens) == 0 {
	return ast.Document(docGroup.RawToken(), nil), nil
	}

while the DocumentNode.String method skips the nil check on body:

go-yaml/ast/ast.go

Line 578 in 3399084

doc = append(doc, d.Body.String())

To Reproduce

Please provide a minimum yaml content that can be reproduced.
We are more than happy to use Go Playground

Expected behavior

.String() should not panic for an empty document node

Screenshots
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Version Variables

• Go version: [e.g. 1.21 ]
• go-yaml's Version: [e.g. v1.11.1 ]

Additional context
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