When a value is coded null in a map, it is not displayed in the tooltip. I usually code missing values with null values (null), not zero values (0).
See https://jsfiddle.net/martindfurrer/xakgs53o/9/

I propose to allow null values in map, mabbubble and mappoint graph types, and display them in the tooltip and data labels, similar to what is already possible in heatmap graphs, for example.

This would elegantly allow to distinguish null values from zero values, and display them correctly.