Fix regression in geom_sf()

[teunbrand]

This PR aims to fix #6189.

Briefly, ggraph::GeomEdgeSf did not have a stroke aesthetic in GeomEdgeSf$default_aes.
That causes some calculations to go awry in GeomSf$draw_panel().
This PR makes a fallback for when stroke is missing.

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

gr <- sfnetworks::as_sfnetwork(sfnetworks::roxel)
ggraph(gr, 'sf') + geom_edge_sf()

^{Created on 2024-11-19 with reprex v2.1.1}