Previously, empty list items (e.g., `1.`) when the marker was not
followed by a space, did not parse as a list-item (or list), but
rather as a paragraph.

This commit adds support and tests for empty list items when not
followed by white-space, when in commonmark mode.

Closes GH-327.
Closes GH-328.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Co-authored-by: Titus Wormer <tituswormer@gmail.com>