Fix regression from #5967

[teunbrand]

This PR fixes a regression from #5967 that is detectable as:

ggplot2/revdep/problems.md

Lines 155 to 156 in d31c051

	Error in if (new_name %in% existing) { : argument is of length zero
	Calls: plot_history ... add_ggplot -> ggplot_add -> ggplot_add.Layer -> new_layer_names

Briefly, this error occurs when the constructor call doesn't get captured cleanly, like in the following case:

library(ggplot2)
rlang::call_name(do.call(geom_point, list())$constructor)
#> NULL

^{Created on 2024-09-10 with reprex v2.1.1}

In this PR, we construct a fallback name based on the geom's ggproto class.

[teunbrand]

Thanks for the review!