Polyfill: Require newDays argument in AdjustDateDurationRecord

[catamorphism]

AdjustDateDurationRecord is always called with a non-undefined newDays, so add an assertion to that effect and remove the ?? operator for days.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.76%. Comparing base (03f9057) to head (b74fed6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3229   +/-   ##
=======================================
  Coverage   96.75%   96.76%           
=======================================
  Files          22       22           
  Lines       10398    10399    +1     
  Branches     1859     1858    -1     
=======================================
+ Hits        10061    10063    +2     
+ Misses        289      288    -1     
  Partials       48       48           

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