Fix geom_ribbon(na.rm)

[teunbrand]

This PR aims to fix #6243.

Briefly, missing values are thrown out and warn about when na.rm = FALSE.
The warning is skipped when na.rm = TRUE but missing values are thrown out.
When throwing out missing values, the group aesthetic is reformed so that groups with NA values break up into new uninterrupted groups.

Reprex from issue, note gradient is uniform now:

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

df <- data.frame(
  x = c(1:3, 1:3),
  ymin = c(1, 1, 1, NA, 2, 2),
  ymax = c(1.5, 2, 1.5, 2.5, 3, 2.5),
  group = c(1,1,1,2,2,2)
)

ggplot(df, aes(x, ymin = ymin, ymax = ymax, group = group, fill = x)) +
  geom_ribbon() +
  scale_fill_viridis_c()
#> Warning: Removed 1 row containing missing values or values outside the scale range
#> (`geom_ribbon()`).

^{Created on 2024-12-17 with reprex v2.1.1}

[thomasp85]

LGTM