BC break: yaml construct error on empty/comment-only stream

[andig]

After upgrading to master we're seing this error when parsing empty/ comment-only yaml:

evcc-io/evcc#29672

What's striking here is:

• error on empty document
• error on comment-only document
• strange error message containing <unknown position>

[andig]

The error on empty input is internal/libyaml.LoadErrors. It seems this can't be type-asserted due to the internal nature:

func TestYamlEmpty(t *testing.T) {
	var res map[string]any
	err := yaml.Unmarshal([]byte(""), &res)
	require.ErrorContains(t, err, "no documents in stream")
}

func TestYamlCommentsOnly(t *testing.T) {
	b := `# just a comment
# another comment
`
	var res map[string]any
	err := yaml.Unmarshal([]byte(b), &res)
	require.ErrorContains(t, err, "no documents in stream")
}