Code clean-up for demographics table

[alex-lauer]

Summary

Hi @rich-iannone,

had to open this PR because I finally found a way to get rid of my last magrittr pipe in the context of purrr and yet still keep my code structure kind of clean:

### Changed 

adsl_summary <- purrr::[map_df](https://purrr.tidyverse.org/reference/map_dfr.html)(
      .x = dplyr::[vars](https://dplyr.tidyverse.org/reference/vars.html)(AGE, AAGEGR1, SEX, ETHNIC, BLBMI),
      .f = \(x) custom_summary(df = dplyr::[filter](https://dplyr.tidyverse.org/reference/filter.html)(rx_adsl, ITTFL == "Y"), group_var = TRTA, sum_var = !!x)
    )

### to:
adsl_summary <- dplyr::filter(rx_adsl, ITTFL == "Y") |> 
  (\(data) purrr::map_df(
    .x = dplyr::vars(AGE, AAGEGR1, SEX, ETHNIC, BLBMI),
    .f = \(x) custom_summary(df = data, group_var = TRTA, sum_var = !!x)
  ))()

Checklist

• [ x ] I understand and agree to the Code of Conduct.
• [ x ] I have listed any major changes in the NEWS.
• [ x ] I have added testthat unit tests to tests/testthat for any new functionality.

[alex-lauer]

The first piece of code was copied from the article in dev. Sorry for the display.

[rich-iannone]

lgtm!

[rich-iannone]

Merging now. Thank you!