plot functions print NULL to console

Both plot_survival() and plot_stress() print NULL to the console. They should return it invisibly instead.

Reprex:

library(stressaddition)
mod <- ecxsys(
    concentration = c(0, 0.0001, 0.001, 0.01, 0.1, 0.316, 1, 3.16),
    survival_tox_observed = c(48.89, 20, 16.67, 0, 6.67, 0, 0, 0),
    survival_max = 100,
    hormesis_concentration = 0.1
)
plot_survival(mod)
plot_stress(mod)