PERF: Let the gradient scales handle missing values

[zeehio]

The last PR related to:

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

Depends on:

• Add na.color support to gradient_n_pal, div_gradient_pal, seq_gradient_pal r-lib/scales#371

Provides a performance improvement with:

• Extend palette contract: Palette capabilities #5032

This pull request passes the na.value colour to the palette in scale_colour_gradient() and friends. This is useful if the two pull requests above get merged, because it will let ggplot2 skip checking for missing values after mapping values to colours (the palette will have already replaced them)

It would require to update the scales version dependency