Don't silently drop NA levels in strings2factors()

[EmilHvitfeldt]

Bug first discovered in https://github.com/tidymodels/recipes/issues/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>