🧹 [Code Health] Simplify optional string empty checks in validate.rs

[ffalcinelli]

🎯 What: Simplified redundant optional string empty checks in src/validate.rs.
💡 Why: To improve code maintainability and readability by using standard idiomatic Rust patterns (unwrap_or_default()).
✅ Verification: Ran cargo fmt, cargo clippy, and the full test suite. Confirmed the logic maintains the same exact behavior.
✨ Result: A cleaner and more idiomatic handling of optional empty string validation.

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PR created automatically by Jules for task 7767770981036107021 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.89%. Comparing base (a6a4e1a) to head (907407d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   89.89%   89.89%           
=======================================
  Files          38       38           
  Lines        2523     2523           
=======================================
  Hits         2268     2268           
  Misses        255      255           

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