fix(manifest): incorrect css entry files in manifest when falsy cssCodeSplit

[sun0day]

Description

fix #14271

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[silverwind]

Maybe also fixes #14047?

[sun0day]

Maybe also fixes #14047?

I am afraid that's a another problem. I will dig into it later.

[sapphi-red]

Thanks for the PR. Closing as css entries doesn't make sense with cssCodeSplit: false (#14271 (comment)).