fix: address mypy error by using "" when default_language is None

[joanise]

Quick fix to resolve mypy errors on main branch.

See also EveryVoiceTTS/EveryVoice#736

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
fs2/cli/synthesize.py	45% smaller

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.49%. Comparing base (9f04b42) to head (b647ec5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   49.59%   49.49%   -0.11%     
==========================================
  Files          27       27              
  Lines        1968     1964       -4     
==========================================
- Hits          976      972       -4     
  Misses        992      992              

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