Tasklists with nameless checkboxes move checkbox to next paragraph

[BenjaminRi]

Suppose you have the following markdown code, which contains a nameless checkbox:

* [ ] 
---
bar

So, we run:

printf '* [ ] \n---\nbar' | cargo run -- --enable-tasklists

This results in:

<ul>
<li></li>
</ul>
<hr />
<p><input disabled="" type="checkbox"/>
bar</p>

Which looks like a stray checkbox after the horizontal rule:

The checkbox was erroneously added to the next paragraph element.

Additionally, if there is no paragraph following, the checkboxes disappear completely:

printf '* [ ] \n* [ ] \n* [ ] \n' | cargo run -- --enable-tasklists

<ul>
<li></li>
<li></li>
<li></li>
</ul>

It seems to me like this behavior was introduced after #557 was fixed.