fix(csv): remove undefined from possible value type of parse result

[magurotuna]

As we discussed in #5605 (comment), it seems like we never get undefined as a parse result of fields. If there is a mismatch in the number of fields across rows, the parse just throws an error.
To better reflect this in typing, this commit removes undefined from the record value type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.37%. Comparing base (b0f2088) to head (a2491b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5617   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files         466      466           
  Lines       37587    37587           
  Branches     5542     5542           
=======================================
  Hits        36226    36226           
  Misses       1319     1319           
  Partials       42       42           

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