Transparent fill of panel borders

[teunbrand]

This PR aims to fix #5782.

Briefly, whenever the panel border is rendered with element_render(), a fill = NA is passed to set a transparent fill regardless of the element settings.

Reprex from linked issue:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme(panel.border = element_rect())

^{Created on 2024-03-19 with reprex v2.1.0}

[thomasp85]

LGTM