fix redistribute_null_units

[Yunuuuu]

fix #6487

[Yunuuuu]

  library(ggplot2)
  library(patchwork)
  p1 <- ggplot(mtcars) +
    geom_point(aes(mpg, disp)) +
    ggtitle("Plot 1")

  p3 <- ggplot(mtcars) +
    geom_point(aes(hp, wt, colour = mpg)) +
    ggtitle("Plot 3")
  p <- patchwork::wrap_plots(
    p1, p3 + scale_color_continuous(guide = guide_colorbar(
      theme = theme(legend.key.height = unit(1, "null"))
    )),
    ncol = 1L,
    guides = "collect"
  )
  print(p)

[teunbrand]

Thank you for the PR! I like that this PR doesn't complicate the logic, however, I don't think it preserves some behaviour I care about (but haven't bothered to embody in a test yet).

In this example, the spacing between the legend should be around the same height as the guide_legend() (because legend has 3 keys, each of 1null height and we're using 3null spacing).
The following example is rendered using this PR's strategy:

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = cty, fill = hwy, shape = drv)) +
  guides(fill = "legend")

p + theme(legend.spacing.y = unit(3, "null"), legend.key.height = unit(1, "null"))

But it should look like this:

^{Created on 2025-05-30 with reprex v2.1.1}