Fix draw_key_rect() not using data$fill for glyph fill

[mitchelloharawild]

This PR resolves #6608:

library(ggplot2)

# Column chart
mtcars |> 
  ggplot(aes(x = factor(cyl), fill = factor(cyl))) +
  geom_col(aes(y = mpg), key_glyph = draw_key_rect)

^{Created on 2025-09-12 with reprex v2.1.1}

As an aside, the pattern of checking if a value is NULL or NA seems to be a common one in defaulting aesthetics that could be wrapped up into a helpful infix function, e.g. x %NA0% y to use y if x is NA or empty. Looking at the code here, it seems that if both fill and colour are NA, then "grey20" may not be used?

[teunbrand]

Thank you for the PR Mitchell!
This looks good to me, can I get you to accept the snapshot change?
The change just reflects the updated logic, nothing wrong with it.

Also if you want to you can add your handle in the NEWS item.

[mitchelloharawild]

Thanks, I've updated the PR.

[teunbrand]

Great, thank you for the contribution!