`fct_unique()` drops `NA` data if it isn't an explicit level

[DavisVaughan]

This came up as an issue in tidyr (tidyverse/tidyr#1275, tidyverse/tidyr#1276).

library(forcats)

x <- factor(c(NA, "x"))

fct_unique(x)
#> [1] x
#> Levels: x

# Retains NA data at the end without adding a new factor level
x <- factor(c(NA, "x"))
tidyr:::fct_unique(x)
#> [1] x    <NA>
#> Levels: x

# If an NA factor level already existed, doesn't change the level position
x <- factor(levels = c(NA, "x"), exclude = NULL)
tidyr:::fct_unique(x)
#> [1] <NA> x   
#> Levels: <NA> x

We could use the tidyr implementation here:

fct_unique <- function(x) {
   if (!is.factor(x)) {
     abort("`x` must be a factor.")
   }

   levels <- levels(x)
   out <- levels

   if (!anyNA(levels) && anyNA(x)) {
     out <- c(out, NA_character_)
   }

   factor(out, levels = levels, exclude = NULL, ordered = is.ordered(x))
 }