I am afraid there is no workaround to suppress the extra line for the time being.
If you apply a border to a text paragraph, PD4ML outputs the paragraph different way: as a table with a single cell. The issue you reported obviously impacts RTF table cells output in general. And as far as I remember, a removal of the line strangely corrupts RTF layout, especially by a table nesting.
If the issue is critical for you, we could address it again.
BTW: did you try PD4ML v4.0.8? Is the issue still there?