Fix 6088

[teunbrand]

This PR aims to fix #6088.

Briefly, #5789 introduced an early exit but this unfortunately happened after data had already been transformed.
This PR performs the check earlier, and encodes this as var = NULL, which will early exit subsequent functions without data transformation.

Reprex from issue:

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

data.frame(
  x = c(-1, 1),
  y = c("x", "y")
) |> 
  ggplot(aes(x, y)) +
  geom_col()

^{Created on 2024-09-16 with reprex v2.1.1}

[thomasp85]

LGTM