Limits of binned scales when data has zero width

[teunbrand]

This PR aims to fix #5066.

Briefly, when the default limits of binned scales have zero-width, they are expanded to have 0.1 width (the same as default_expansion()).

Reprexes from the linked issue:

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

data.frame(x = c("a", "b"), 
           y = c("A", "B")) |> 
  ggplot() + 
  geom_bin_2d(aes(x = x, y = y)) + 
  scale_fill_binned()

ggplot(data.frame(x=1), aes(x=x)) + geom_bar() + scale_x_binned()

^{Created on 2024-12-10 with reprex v2.1.1}

[thomasp85]

LGTM