fix: panic when concat external module request is undefined

[ahabhgk]

Summary

fix #10361

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rspack canceled.

Name	Link
🔨 Latest commit	13002b1
🔍 Latest deploy log	https://app.netlify.com/projects/rspack/deploys/6846862524b6e400080a60bb

[codspeed-hq]

CodSpeed Performance Report

Merging #10611 will not alter performance

_{Comparing fix-external-concat (13002b1) with main (c96a07f)}

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Summary

✅ 12 untouched benchmarks