Fix geom_ribbon(stat = "align") dropping single groups

[teunbrand]

This PR aims to fix #6680.

The issue was caused by single groups that don't get assigned an align_padding computed aesthetic, because of the early exit here:

ggplot2/R/stat-align.R

Lines 19 to 21 in 081806c

	if (is_unique(data$group)) {
	return(data)
	}

These would get NA values for align_padding when combined with other panels, which this PR now keeps around.

Reprex from issue:

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

df <- data.frame(
  x = c(1, 2, 1, 2, 1, 2),
  y = c(1, 1, 1, 1, 1, 1),
  g = c("A", "A", "B", "B", "C", "C"),
  f = c("x", "x", "x", "x", "y", "y")
)

ggplot(df, aes(x, y, fill = g)) +
  geom_area(position = "stack") +
  facet_wrap(vars(f))

^{Created on 2025-10-08 with reprex v2.1.1}