Remove shape as non-missing aes in geom_segment()

[teunbrand]

This PR aims to fix #5517.

geom_segment() no longer drops rows with missing shape aesthetic, as shape doesn't actually correspond to a property of the line that is being drawn.

Reprex from #5517:

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

df <- data.frame(
  x = 0, 
  xend = 1, 
  y = c(0, 1), 
  yend = c(1, 0), 
  shape = c("A", NA)
)

ggplot(df, aes(x, y, xend = xend, yend = yend)) +
  geom_segment(aes(shape = shape))
#> Warning in geom_segment(aes(shape = shape)): Ignoring unknown aesthetics: shape

^{Created on 2023-11-29 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks for the review Thomas!