Add default value to diseasyoption

[RasmusSkytte]

Intent

This small PR adds a default value to diseasyoption via the .default argument.
(inspired by purrr::pluck())

Approach

N/A

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (7ccc836) to head (0c01463).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   96.29%   96.39%   +0.10%     
==========================================
  Files          13       13              
  Lines         702      722      +20     
==========================================
+ Hits          676      696      +20     
  Misses         26       26              

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