Fix failure when adding a row to an empty masked Table

Also fixed another issue where (with masked table) calling
add_row() would add a row copying the first row instead of the
advertised behavior of adding a row of "zeros".  The docstring
was clear that it should be zeros.
Mark test values as byte strings for Python 3
Merge pull request #973 from taldcroft/table/empty-add-row

Add_row fails for empty masked table