Require x *or* y in stat_ydensity()

[teunbrand]

This PR aims to fix #6600.

Briefly, we initiate x = 0 or y = 0 when they are missing, similar to stat_boxplot().
Examples based on the issue for both orientations:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
df <- data.frame(x = rnorm(100))

ggplot(df, aes(x = x)) +
  geom_violin()

ggplot(df, aes(y = x)) +
  geom_violin()

^{Created on 2025-09-24 with reprex v2.1.1}