fix(_internal/compareValues): place null and undefined values last when sorting

[sukvvon]

Summary

• Add null/undefined handling to compareValues in src/_internal/compareValues.ts
• null and undefined values are placed at the end for ascending order, at the beginning for descending order
• undefined is placed after null (matching lodash behavior)
• Add regression tests for both sortBy and orderBy

Fixes #1213

Test Plan

• sortBy with null/undefined values places them at the end
• orderBy ascending places null/undefined at the end
• orderBy descending places undefined before null at the beginning
• All existing sortBy/orderBy tests pass
• Full test suite passes (4334 tests)

[vercel]

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview, Comment	Apr 3, 2026 11:20am

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.97%. Comparing base (cd553ee) to head (cfc23f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1681   +/-   ##
=======================================
  Coverage   99.97%   99.97%           
=======================================
  Files         497      497           
  Lines        4662     4675   +13     
  Branches     1348     1353    +5     
=======================================
+ Hits         4661     4674   +13     
  Misses          1        1           

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