perf: Don't traverse file list twice for extension validation

[nameexhaustion]

Also fixes a panic when the expanded paths were empty

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.89%. Comparing base (aa3b2c3) to head (5df1754).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18620      +/-   ##
==========================================
- Coverage   79.94%   79.89%   -0.05%     
==========================================
  Files        1505     1506       +1     
  Lines      202579   203071     +492     
  Branches     2873     2889      +16     
==========================================
+ Hits       161958   162253     +295     
- Misses      40073    40268     +195     
- Partials      548      550       +2     

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[ritchie46]

Can you rebase?

[codspeed-hq]

CodSpeed Performance Report

Merging #18620 will degrade performances by 30.68%

_{Comparing nameexhaustion:empty-expansion (5df1754) with main (aa3b2c3)}

Summary

❌ 1 regressions
✅ 38 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	nameexhaustion:empty-expansion	Change
❌	test_groupby_h2oai_q1	2.1 ms	3.1 ms	-30.68%