`grattan_richlegend()` errors in faceted plots when the legend data doesn't cover every panel

grattan_richlegend() fails to render whenever it's used in a faceted plot and its layer data is present in only some panels (the normal case — you usually want the legend in a single facet). Instead of drawing the legend in the panels that have data and nothing in the others, it errors out during grob construction.

The error comes from grid:

Error in `grattan_richlegend()`:
! Problem while converting geom to grob.
ℹ Error occurred in the Nth layer.
Caused by error in `checkNA()`:
! mixture of missing and non-missing values for fontsize

As an example:

library(grattantheme)
library(ggplot2)
library(dplyr)

d <- tibble(
  facet = rep(c("a", "b"), each = 4),
  x = rep(1:4, 2),
  y = c(1, 2, 3, 4, 2, 3, 4, 5),
  g = rep(c("G1", "G2"), 4)
)

# Legend data only for facet "a"
leg <- tibble(facet = "a", g = c("G1", "G2"))

ggplot(d, aes(x, y, colour = g)) +
  geom_line(aes(group = g)) +
  grattan_richlegend(
    data = leg,
    aes(colour = g, label = g),
    legend.position = "topleft",
    inherit.aes = FALSE
  ) +
  facet_wrap(~facet) +
  scale_colour_manual(values = c(G1 = grattan_orange, G2 = grattan_red),
                      guide = "none") +
  theme_grattan()
#> Error ... checkNA(): mixture of missing and non-missing values for fontsize

Ideally grattan_richlegend should be able to write to one single facet, defaulting to the top left, but changeable by the user.

Reproduce the faceted-plot example in the issue and start at the grattan_richlegend() entry point, following its grob construction for panels with and without legend data. Done means the legend renders in the panel containing data, other panels remain empty, and the target facet can be changed by the user without the fontsize error.