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

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

Start by reproducing the three Markdown list examples with commonmark.js and compare the rendered paragraph spacing. Trace the parser and renderer behavior for empty list items in loose lists. Done means empty items are treated as empty paragraphs so all items follow the input list's tight or loose spacing consistently.