Unfortunately, it is not possible to show a tooltip for null points in line series, since these points simply do not exist on the chart. This functionality only works for charts like heatmap, which displays empty tiles for null value points.
In that case, if you would like to display the tooltip - you would need to create a visible point so that the user can hover over it.
In case of any other questions, please do not hesitate to contact us anytime,
Best regards!