When you don't "touch" the cell (with "touch" I mean selecting a kind of cell in the UI), the dict containing the value does not exist... so, with this function: https://github.com/ipython/nbconvert/pull/69/files#L2R65 I set the name "untouched" capturing the exception.
When you "touch" the cell (for example making it "slide", but then you can go back a mark it as none) the slideshow dict is created and it takes the "-" value for none cells.
I think that this behavior is because the current implementation of slideshow.js...