Consider PANEL for handling NAs in line geoms

[teunbrand]

This PR aims to fix #6533.

Briefly, line geoms require special NA-handling methods because we want to exclude terminal vertices but not internal vertices from a path. We use group to determine these, but we should consider PANEL as well.

Reprex from issue:

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

d2 <- data.frame(x = 1:3, col = c("blue", "green", NA)) |> 
  tidyr::crossing(group = c("A", "B"))

ggplot(d2, aes(x, x, color = I(col), group = 1)) +
  facet_wrap(~ group) + 
  geom_line(lwd = 2)
#> Warning: Removed 2 rows containing missing values or values outside the scale range
#> (`geom_line()`).

^{Created on 2025-07-03 with reprex v2.1.1}