empty list item should be treated as an empty paragraph in loose list

[vassudanagunta]

This renders as expected:

- all four of these items will
- be rendered uniformly "tight"
- 
- including these last two

This does not:

- all four of these items will

- be rendered uniformly "loose"

- 

- except these last two

With typical stylesheet spacing of p elements, this one's spacing ends up the "opposite" of the input Markdown:

- all four of these items will
- be rendered uniformly "loose"
- 

- except these last two