Parse: minor optimizations

[thaJeztah]

• add early return for empty strings
• skip lowercase-regex if the input is already lowercase

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.21%. Comparing base (c577ba6) to head (2bae058).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   83.71%   84.21%   +0.49%     
==========================================
  Files           5        5              
  Lines         393      304      -89     
==========================================
- Hits          329      256      -73     
+ Misses         54       38      -16     
  Partials       10       10              

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