List with empty items corner case generates invalid HTML

The following snippet

```
 1. 
 2. 
```

generates

```
<ol>
<li>
<ol>
<ol>
<li>
</ol></li>
</ol></li>
</ol>
```

which is not valid HTML.

GitHub, for example, correctly generates

```
<ol>
    <li></li>
    <li></li>
</ol>
```
