Don't silently drop NA levels in strings2factors()

[EmilHvitfeldt]

Bug first discovered in #1290.

• add tests
• add news

Before

library(recipes)

ex_data <- tibble(
  x = factor(c("a",NA,"c"), exclude = NULL)
)

ex_data$x
#> [1] a    <NA> c   
#> Levels: a c <NA>

rec_res <- recipe(~., data = ex_data) |>
  prep() |>
  bake(new_data = NULL)

rec_res$x
#> [1] a    <NA> c   
#> Levels: a c

After

library(recipes)

ex_data <- tibble(
  x = factor(c("a",NA,"c"), exclude = NULL)
)

ex_data$x
#> [1] a    <NA> c   
#> Levels: a c <NA>

rec_res <- recipe(~., data = ex_data) |>
  prep() |>
  bake(new_data = NULL)

rec_res$x
#> [1] a    <NA> c   
#> Levels: a c <NA>

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex https://reprex.tidyverse.org) and link to this issue.