colour_ramp already supports na.color, so this pull request just adds the ability to pass it to gradient_n_pal and friends

• scales/R/colour-ramp.R

  Line 35 in b3df2fb

  colour_ramp <- function(colors, na.color = NA, alpha = TRUE) {

This is related to:

• geom_raster() of a matrix: Performance analysis and improvements tidyverse/ggplot2#4989

Proposed NEWS entry:

 * `gradient_n_pal`, `div_gradient_pal` and `seq_gradient_pal` accept a `na.color` (#, @zeehio)