Fix a Remarkup issue where empty list items render with markup in items

Summary:
When you type this:

- A
-

..it currently displays as:

* A
* -

That is, the second bullet has a "-" next to it. Instead it should display like this:

* A
*

Make it do that.

Test Plan: {F26810}

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4134