It can be helpful when writing other functions to just return the names
of the variables that contain missing values. miss_var_which returns a
vector of variable names that contain missings. It will return NULL when
there are no missings.
miss_var_which(data)character vector of variable names
miss_var_which(airquality)
#> [1] "Ozone" "Solar.R"
miss_var_which(mtcars)
#> NULL