Error in TooltipRenderOp.render when a NaN value is selected

[OptimisticSean]

If TooltipRenderOp.anchor is null, TooltipRenderOp.render will try to find the anchor point based on the selected data, which will cause an error since it's NaN. TooltipRenderOp.render should probably return early and skip creating the tooltip when anchorRst.dx or anchorRst.dy is NaN.