fix for rappture automatic ticket #256452. if we receive no data in the xy
curve object, we don't add empty data to the list of curves. this also affects
xvector and yvector objects that have unequal lengths. if this is encountered,
we don't add the curve to the list of curves. we also no not tell the user
there was an error.
adding file nodata.xml which showed the error described in the automatic
ticket.
