Fix Empty Lines Between List Markers and Checklists Not Including Lines Made Up of Just Whitespace

[pjkaufman]

Fixes #1354

There was a reported issue where empty lines was not including lines with just whitespace. In order to handle this, we need to make sure that to include that in the regex logic.

Changes Made:

• Updated the regex to include whitespace only lines
• Added a test case for the scenario in question