chore(schema): Remove explicit "n/a" from column definitions

[effigies]

Small cleanup to simplify column definitions. n/a is valid in ALL columns in a TSV file. We could either add it as an explicitly permitted value to all columns or leave it implicit. I'm going with implicit because it makes for cleaner definitions.

[tsalo]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.10%. Comparing base (7b0f76d) to head (dd708ba).
Report is 263 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1903   +/-   ##
=======================================
  Coverage   88.10%   88.10%           
=======================================
  Files          16       16           
  Lines        1396     1396           
=======================================
  Hits         1230     1230           
  Misses        166      166           

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