Fixing nan_color for CairoMakie when plotting heatmaps

[ph-kev]

Since you can provide your own masking function that NaN out data, you may want to change the color of the NaNs to something else. With the CairoMakie backend, you can't easily change it by using the nan_color keyword. See MakieOrg/Makie.jl#4524.

To fix this, we can do what is suggested in that issue and plot it like this: surface!(ax, lons, lats, zeros(size(field)); color = field, shading = NoShading, nan_color = :red). Here, we are using the color keyword.