fix: tailwind css sourcemap warning

[sapphi-red]

Description

#7432

fixes Sourcemap for "/path/to/tailwind.css" points to missing source files. with postcss virtual files.

Additional context

It seems tailwind returns <input css foobar> as a path in sourcemap.sources.
Since postcss uses <no source>, I thought postcss uses <something> to indicate a virtual file.
Though I did not find any documents about it.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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