Stack divide zero

[teunbrand]

This PR aims to fix #6338.

Briefly, it prevents position_fill() from "filling" when all observations are zero, which would previously generate NAs through division by zero.

Reprex from the issue, note the lack of warning about missing values:

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

df <-
  tibble::tribble(
    ~season,   ~family, ~count,
    "Summer",    "Bird",    10,
    "Summer",    "Fish",    12,
    "Winter",    "Bird",     4,
    "Winter",    "Fish",     1
  )

df |>
  ggplot(aes(x = season, y = count, fill = family, group = family)) +
  geom_area(position = position_fill())

^{Created on 2025-02-19 with reprex v2.1.1}