Continue checkbox lists with empty checkbox

Currently, if the last item was a marked checkbox, the next item will be a marked checkbox. But I don't think this is the expected behaviour.

Adding a new line after this:

```markdown
- [ ] Item 1
- [X] Item 2
```

currently produces this:

```markdown
- [ ] Item 1
- [X] Item 2
- [X] 
```

when it should produce this:

```markdown
- [ ] Item 1
- [X] Item 2
- [ ] 
```