fix(scheme): reject an empty column in --scheme

[bhdnb]

--scheme en= silently mapped en to column 0 — the first real column — instead of erroring, so a typo like --scheme en=,de=1 mis-mapped the file rather than failing. Number('') is 0, an integer and not negative, so an empty column cleared the validation guard.

Affects tm upload and glossary upload, the two callers of parseScheme. en=0 still parses — '0' is truthy, so widening the check to !column doesn't swallow a legitimate first column.

parseScheme's unit tests live on next_tests branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.21%. Comparing base (323f860) to head (11e8944).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1114   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         142      142           
  Lines       11494    11494           
=======================================
  Hits        10598    10598           
  Misses        896      896           

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