Follow-up: Skip stylesheets with empty href attributes in Site Health audit

[westonruter]

Summary

Follow-up to #2281.
See #2278.

Use of AI Tools

This addresses these Copilot suggestions: #2281 (review)

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.87%. Comparing base (1f31042) to head (fcb7ba7).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2328   +/-   ##
=======================================
  Coverage   68.87%   68.87%           
=======================================
  Files          90       90           
  Lines        7618     7618           
=======================================
  Hits         5247     5247           
  Misses       2371     2371           

Flag	Coverage Δ
multisite	68.87% <ø> (ø)
single	35.41% <ø> (ø)

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