Please explain me.
It's not a bug. Every text container uses at least 1 line, because it contains at least 1 insertion point.
Interestingly, an empty cell contains one line . . . and zero paragraph!
You may also observe that the single line of an empty container has no character:
myEmptyCell.lines.characters.length == 0
(but you have: myEmptyCell.lines.insertionPoints.length == 1 )