Domesticate NULL aesthetic labels

[teunbrand]

This PR aims to fix #6616.

Briefly, whenever a (default) label is derived from a NULL aesthetic, we set the fallback = TRUE attribute. This allows labels derived from other layers to overrule that label.

Reprex from the issue, notice that we have cty and hwy labels instead of x/y labels.

devtools::load_all("~/packages/ggplot2/")

ggplot(mpg) +
  aes(cty, hwy) +
  geom_rect(
    data = data.frame(xmin = 11, xmax = 35, ymin = 17, ymax = 44),
    map = aes(
      x = NULL, y = NULL,
      xmin = xmin, xmax = xmax, 
      ymin = ymin, ymax = ymax
    )
  ) +
  geom_point()

^{Created on 2025-09-24 with reprex v2.1.1}