fix: respect theme(strip.placement) when strip position is ambiguous

[Yunuuuu]

fix#446

This PR ensures that add_strips() respects the strip.placement setting in the theme.

library(ggplot2)
set_theme(theme_classic() + theme(strip.placement="outside"))
p1<- ggplot(mtcars) + geom_point(aes(mpg, disp))
p2<- ggplot(mtcars) + geom_boxplot(aes(gear, disp, group=gear))
p3<- ggplot(mtcars) + geom_bar(aes(gear)) + facet_wrap(~cyl)
p1/ (p3|p2)

[Yunuuuu]

I’m not sure why the PR failed the GitHub Actions check, since it succeeds on my local machine. I also noticed that all tests were already failing before this PR.