Empty plot flag for ab/h/v-line

[zeileis]

When I want to add ab/h/v-lines in histograms I can do something like:

tinyplot(~ body_mass, data = penguins, type = "histogram")
tinyplot_add(type = type_vline(4500), lwd = 2, col = 2)

However, when I use the two-sided formula body_mass ~ 1 instead of the one-sided ~ body_mass, then I get the automatic dispatch to the histogram but the ab/h/v-line is ignored:

tinyplot(body_mass ~ 1, data = penguins)
tinyplot_add(type = type_vline(4500), lwd = 2, col = 2) ## ignored

The reason seems to be that tinyplot.default() infers the display to be an empty_plot in this code:

https://github.com/grantmcdermott/tinyplot/blob/main/R/tinyplot.R#L1501-L1505

Because "histogram" is explicitly exempt in the empty_plot rule, the histogram works. But the ab/h/v-lines are not drawn at all.

I guess we could (or should?) explicitly exempt ab/h/v-line as well? But maybe the empty_plot should also correctly catch y ~ 1 displays in general?