Tiptoe around na.value

[teunbrand]

This PR aims to fix #6499.

The solution is the same as in #6474.
Reprex from the issue:

devtools::load_all("~/packages/ggplot2/")

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  scale_colour_manual(
    values = structure(c(`4` = "blue", f = "red", r = "green"), class = "foobar"),
    na.value = "gray"
  )

^{Created on 2025-06-11 with reprex v2.1.1}

[thomasp85]

Small comment but otherwise LGTM