bugfix in perSecond():  if the series has null data, diff the existing data.
    Consider a series that looks like this:
        X1,null,X2,null,X3,null,...
    In the current implmenentation, perSecond() will return all nulls
    for this series.  With this bugfix, perSecond will return instead:
        (x2-x1)/step,null,(x3-x2)/step,null,...