🚑 Repair layer names that might have become lost

[teunbrand]

This PR aims to fix #6295.

It bequeaths the 'unknown' name upon layers that have lost their names.
Reprex from issue:

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

# To wipe out names
p$layers <- list(geom_path(), geom_path())
p <- p + geom_point() + geom_point()
names(p$layers)
#> [1] "unknown...1"    "unknown...2"    "geom_point"     "geom_point...4"

^{Created on 2025-01-22 with reprex v2.1.1}

[thomasp85]

LGTM