Handle empty string values in command generators

[GitBib]

• Added logic in command generators to handle empty string values for attachment names, descriptions, MIME types, track languages, and titles.
• Included additional test cases to ensure proper handling of these scenarios.
• Updated documentation with doctest skip markers and refined type checks.

[GitBib]

fix #97

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (33a2a67) to head (fb2ed53).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #99   +/-   ##
=======================================
  Coverage   88.52%   88.52%           
=======================================
  Files          21       21           
  Lines        1290     1290           
=======================================
  Hits         1142     1142           
  Misses        148      148           

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