Return an array and missing value suitable for factorization.
An array suitable for factorization. This should maintain order and be a supported dtype (Float64, Int64, UInt64, String, Object). By default, the extension array is cast to object dtype.
The value in values to consider missing. This will be treated as NA in the factorization routines, so it will be coded as -1 and not included in uniques. By default,