Colors used: min max base empty
When legendColors is not specified, cal-heatmaps falls back to the default color scheme. There, cells with value 0 get the minimal color of the legend scale and cells with value null get the base color, grey.
When legendColors is specified with base and empty, cells with value null should have the base color, while cells with value 0 have the empty color.
When legendColors is specified but base is not given, cells with value null should have the default base color (grey), while cells with value 0 have the empty color.
In this case, both cells with value null and 0 should be colored