Apply review comments for derive_blfl

[ShiyuC]

This PR addresses the comments from #95 (review).

• Place Closes #<insert_issue_number> into the beginning of your Pull z
  Request Title (Use Edit button in top-right if you need to update)
• Code is formatted according to the
  tidyverse style guide. Run
  styler::style_file() to style R and Rmd files
• Updated relevant unit tests or have written new unit tests, which should
  consider realistic data scenarios and edge cases, e.g. empty datasets, errors,
  boundary cases etc. - See
  Unit Test Guide
• If you removed/replaced any function and/or function parameters, did you
  fully follow the
  deprecation guidance?
• Update to all relevant roxygen headers and examples, including keywords
  and families. Refer to the
  categorization of functions to tag appropriate keyword/family.
• Run devtools::document() so all .Rd files in the man folder and the
  NAMESPACE file in the project root are updated appropriately
• Address any updates needed for vignettes and/or templates
• Update NEWS.md if the changes pertain to a user-facing function (i.e. it
  has an @export tag) or documentation aimed at users (rather than developers)
• Build oak site pkgdown::build_site() and check that all affected
  examples are displayed correctly and that all new functions occur on the "Reference" page.
• Address or fix all lintr warnings and errors - lintr::lint_package()
• Run R CMD check locally and address all errors and warnings - devtools::check()
• Link the issue in the Development Section on the right hand side.
• Address all merge conflicts and resolve appropriately
• Pat yourself on the back for a job well done! Much love to your accomplishment!

[github-actions]

Package	Line Rate	Health
sdtm.oak	94%	✔
Summary	94% (977 / 1044)	✔

[ramiromagno]

Thanks @ShiyuC, looks great!