Show NA keys better

[teunbrand]

This PR aims to fix #5749.

Briefly, when including NA as part of discrete layers, the legend checks for NA values in the data. What the legend didn't consider, is that the values might not be NA themselves but map to NA for some reason. This PR tries to detect such cases by checking if any values in the data are not included in the breaks.

Reprex from #5749 (comment)

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

penguins |>
  ggplot() +
  geom_point(
    aes(x = flipper_length_mm, 
        y = body_mass_g,
        col = species)) +
  scale_colour_manual(
    values = c("red", "blue"), 
    limits = c("Adelie", "Gentoo", NA)
  )
#> Warning: Removed 2 rows containing missing values or values outside the scale range
#> (`geom_point()`).

^{Created on 2024-03-07 with reprex v2.1.0}

[thomasp85]

LGTM