Consider a VMerge value of "continue" when determining if a cell is a dummy

Cell objects only check for an empty value (which I've gathered is how Word implements it(?), but not LibreOffice) and not so if a table contains `<w:vMerge w:val="continue"/>` instead of `<w:vMerge/>`, `Cell.getExtraRows()` returns the wrong value.

This one line commit fixes that.