Need to define `unique.Period()`

[StefanoMezzini]

Problem:
unique() currently fails to return the unique values for {lubridate} Period objects, which causes issues when trying to check data with check_data().

Steps to reproduce this behaviour:

library(lubridate)
#>
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#>
#>     date, intersect, setdiff, union
library(chk)

x <- lubridate::hms(c("00:00:00", "24:00:00", NA_character_))

x
#> [1] "0S"        "24H 0M 0S" NA

unique(x)
#> [1]  0 NA

check_data(data.frame(period = hms("01:02:03")), list(period = x))
#> Error in `lapply()`:
#> ! All elements of `data.frame(period = hms("01:02:03"))$period` must be equal to 0.
#> Use `rlang::last_error()$y` to show the object compared to.
#> Run `rlang::last_trace()` to see where the error occurred.

unique.Period <- function(x, ...) {
  x |>
    as.character(x) |>
    unique() |>
    as.period()
}

unique(x)
#> [1] "0S"        "24H 0M 0S" NA

check_data(data.frame(period = hms("01:02:03")), list(period = x)) # passes

Expected output/desired solution:
See reprex above.

[StefanoMezzini]

the problem is that unique.Period() should really be defined in {lubridate}, since {chk} does not depend on {lubridate}.