Wrong mapping provided to limits that include `NA`

[teunbrand]

Briefly, when including NA in the limits, I interpret that as wanting to display the na.value for it.
This is fine (aside from #5749 (comment)).

library(ggplot2)

p <- ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point()

p + scale_colour_discrete(limits = c("4", "5", "6", NA))

However when you don't have NA as the last limit, anything after it gets the wrong mapping.
The cyl == 8 data become grey instead of blue.

p + scale_colour_discrete(limits = c("4", "5", NA, "8"))

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