Why does utils.appendChild allow an empty parent array?

When writing tests for the utils methods, I came across some logic that was confusing:
https://github.com/benjycui/jsonml.js/blob/master/lib/utils.js#L132

```js
} else if (child || !parent.length) {
// append
parent.push(child);
```

This allows for some unclear behavior, particularly when allowing exception for a parent with no length.

1. append a child to an empty parent array
```js
const jml = [];
utils.appendChild(jml, '');
// jml === ['']
```
```js
const jml = [];
utils.appendChild(jml, ['p', 'hello']);
// jml === [ ['p', 'hello'] ]
```
Is this a real use case or an edge case bug? I'm guessing this would never be desired and should probably not be allowed.

Start at lib/utils.js:132 and inspect the surrounding appendChild logic and its existing tests, if any. Reproduce the empty-parent examples from the issue, then determine the intended behavior for empty parents and document that decision with regression coverage; done means the behavior is explicit and verified.

Written by the indexing model from the issue text.