Hitting enter in an empty sublist creates a paragraph and breaks list structure

### Browsers affected

This was verified in IE7, but might be an issue in other browsers/versions.

### Reproduction Steps

#### List Structure

Note: *|* is the cursor
```
A
B
  1
  2
    *
    *
    *|
 3
C
```

#### Steps

1. Start with the cursor as indicated
2. Type Enter
3. Type Enter
4. Type Enter

#### Unexpected Result

A paragraph tag is created inside the list.

### Technical Details

#### Starting HTML

```
<ol>
<li>1<ol>
<li>1.1</li><li>1.2</li><li>1.3</li></ol></li><li><strong>2</strong> 
<ol>
<li>2.1</li><li>2.2<ul>
<li>2.2.1</li><li>2.2.2</li><li>2.2.3</li></ul></li><li>2.3</li><li>2.4<ul>
<li>2.4.1</li><li>2.4.2</li></ul></li></ol></li><li><strong>3</strong> </li></ol>
```