zero-range limits in binned scale

[teunbrand]

This PR is intended as a partial fix to #5066. In brief, the map() method circumvents cut() when there aren't enough breaks for that function. It resolves the error in #5066, but does not resolve the guide_coloursteps() issue (because that guide is in reconstruction). In choosing a palette-value for zero-range limits, it now behaves similar to scale_fill_continuous().

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

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

b <- a + scale_fill_binned()

a + b

^{Created on 2022-12-01 by the reprex package (v2.0.1)}

[thomasp85]

LGTM