Add na.color support to gradient_n_pal, div_gradient_pal, seq_gradient_pal

[zeehio]

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

• https://github.com/r-lib/scales/blob/b3df2fb6efd5b440377b0699d3830f1082fa3140/R/colour-ramp.R#L35

This is related to:

• https://github.com/tidyverse/ggplot2/issues/4989

Proposed NEWS entry:

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