Zero length breaks no longer work with discrete positional scales

[cpsievert]

With current CRAN version, this code produces:

ggplot(PlantGrowth, aes(x = group, y = weight)) + 
  geom_boxplot() + 
  scale_x_discrete(breaks = NULL)

With GH version (likely introduced by 7e5ff92, cc @thomasp85), this code produces an error:

Error: `mapped_discrete` objects can only be created from numeric vectors

[clauswilke]

I suggest adding a unit test for this case, so the problem won’t reoccur at some point in the future.

[thomasp85]

yeah - you're probably right