This PR aims to fix #5945.

Briefly, a discrete position scale was treated as empty when range data is absent and limits are NULL.
This PR also treats such scale as empty when range data is absent and limits is a function.
In such cases, functions don't recieve proper 'natural data limits' because the range data is absent.
Potentially, this would cause troubles downstream, so I thought it best to treat such cases as empty scales.

Reprex from issue, note the empty plot instead of errors:

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

df <- data.frame(
  level = factor(character(), "1"),
  x = numeric(),
  meanX = numeric()
)

ggplot(df) + 
  geom_col(aes(x = meanX, y = level, fill = level)) +
  labs(x = NULL, y = NULL, fill = NULL, title = "title") +
  scale_y_discrete(labels = NULL, limits = rev, drop = FALSE)

^{Created on 2024-06-19 with reprex v2.1.0}