Preserve NAs in discrete palettes

[teunbrand]

This PR aims to fix #5929.

Briefly, there are 3 situations by which a value can become NA in a discrete scale.

1. The data value is NA.
2. The data value does not match the scale limits.
3. The data value is mapped to NA because a palette value is NA.

Currently, we cannot distinguish between situation (2) and situation (3).
This PR ensures that situation (1) and (2) are laundered through the na.translate/na.value mechanism, but (3) is not because we could assume (3) is intentional by users (see issue).

As a demo, notice that 'e' gets NA due to (3) and is subsequently removed, while 'r' gets na.value due to (2).

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy, colour = fl)) +
  geom_point() +
  scale_colour_manual(
    values = c("tomato", "dodgerblue", NA, "limegreen"),
    limits = function(x) setdiff(x, "r"),
    na.value = "black"
  )
#> Warning: Removed 8 rows containing missing values or values outside the scale range
#> (`geom_point()`).

^{Created on 2024-06-19 with reprex v2.1.0}

[teunbrand]

This part is the trick to discrimating (2) and (3). In case (2) we get nomatch'ed to the last palette value, which we set to the na.value. This leaves (3) as-is.

[teunbrand]

We no longer need to translate is.na(pal_match) because we do so through the (lack of) matching above

[thomasp85]

LGTM