Lowercase dataframe column strings with missing values

[bkamins]

use the passmissing wrapper:

julia> transform(df, :Y => ByRow(passmissing(lowercase)) => :Y)
6×2 DataFrame
 Row │ Y       Z
     │ String  String?
─────┼─────────────────
   1 │ a       A
   2 │ b       missing
   3 │ c       C
   4 │ d       missing
   5 │ e       E
   6 │ f       missing