selecting empty list item causes error

[bantic]

To reproduce:

• create three list items, where the 1st and 3rd have text and the 2nd is blank
• put cursor in the 2nd item, use shift+ to select the list item below
• console reports "Could not find parent section from element node"

This happens because the browser will report (via window.getSelection()) that the focus node is the 3rd <li> itself, rather than the inner textNode, which is what Cursor#fromNode was expecting.