Plotting of data with `null` values doesn't always work

[Vindaar]

Take the old example from the nimib penguins example:

ggplot(df, aes(x="body_mass_g", y="flipper_length_mm", color = "sex", shape="species")) + 
  geom_point() + 
  ggsave("/tmp/broken.png")

using the dataset from here: https://github.com/allisonhorst/palmerpenguins

It gives a runtime error related to missing values. This should ideally work. The issue is probably due to having missing values in a mapping variable, which might not be handled correctly.