Fix for #6812/Interpolate property has issue when the first point is null

[cheewe]

Thanks for the wonderful fix to #6812(​http://bugs.dojotoolkit.org/changeset/27625/dojo). I love it. One behavior that was changed with this latest patch is if the first point is null, nothing is drawn. So in your example, this works exactly what you expected it

var seriesB = [2, 3, null, null, 4, 5, null, 6, 7];

But

var seriesB = [null, 2, 3, null, null, 4, 5, null, 6, 7];

doesn't render at all. In fact,

var seriesB = [null, 2, 3, 4, 5, 6, 7];

rendered before, but not with the latest patch.

[Adam Peller]

Duplicate of #6812.
dup of #6812