fix(sourcemap): sourcemap is incorrect when sourcemap has sources: [null]

[Dunqing]

Description

Fixes: #13657

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[sapphi-red]

Thank you for the PR!

[sapphi-red]

Would you merge this playground to js-sourcemap playground to reduce the number of playgrounds?

[Dunqing]

Sure!

[sapphi-red]

Does combineSourcemaps work without doing the same normalization (i.e. setting [this.filename]) above? This happens when a module is transformed by a plugin then transformed by another plugin that uses MagicString.

[Dunqing]

Sources are absolutely present when entering here Because the first source map has added sources or the sources themselves exist

[sapphi-red]

combineSourcemaps has two inputs. The second one (combinedMap) wouldn't have sources: ['']. But the first one (m) might have sources: [''], no?

[Dunqing]

No, the second one definitely have

[sapphi-red]

I've actually tested it now by changing the code like below and renaming zoo.js to zoo.ts.

          console.log('Before', m, combinedMap)
          combinedMap = combineSourcemaps(cleanUrl(this.filename), [
            m as RawSourceMap,
            combinedMap as RawSourceMap,
          ]) as SourceMap
          console.log('After', combinedMap)

I got the following result:

Before SourceMap {
  version: 3,
  file: undefined,
  sources: [ '' ],
  sourcesContent: undefined,
  names: [],
  mappings: 'AAAA,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC;'
} {
  version: 3,
  sources: [ 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts' ],
  sourcesContent: [ "export const zoo: string = 'zoo'\n" ],
  mappings: 'AAAO,aAAM,MAAc;',
  names: []
}
After SourceMap {
  version: 3,
  mappings: 'AAAO,aAAM,MAAc',
  names: [],
  sourceRoot: undefined,
  sources: [ 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts' ],
  sourcesContent: [ "export const zoo: string = 'zoo'\n" ],
  file: 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts'
}

It seems it works at least for this case.