Modify `add_missing_column` so that can use existing column if exists.

[joethorley]

library(tidyplus)

x <- data.frame(y = 1:2)

add_missing_column(x, z = .data$y)
#> Error in `.data$y`:
#> ! Column `y` not found in `.data`.

^{Created on 2024-07-27 with reprex v2.1.1}