Return undefined loc if declaration missing source

[sibbng]

↪️ Pull Request

Fixes #6898

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[height]

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[DeMoorJasper]

Tests seem to be failing, could you fix them?

[sibbng]

Tests seem to be failing, could you fix them?

Fixed. Remaining failures related to yarn pnp seems persist in v2 branch too.

[mischnic]

Last thing: can you remove the packages/core/integration-tests/test/integration/postcss-plugins-decl/index.js and use index.js directly as the entry point (and remove await run(b))?

[sibbng]

and use index.js directly as the entry point

I guess, you meant to say index.css here.

[mischnic]

Yes 👍