Add "null" to the legend.

[AndreiKingsley]

Now we can specify a value in the scale that will be used for null values, but it would be great if it were displayed in the legend (including in the case of a continuous scale - for example, display the value for null next to the main legend).

[alshan]

ggplot (r) can show n/a values in legend (not sure about colorbar):

https://ggplot2.tidyverse.org/reference/scale_grey.html