Do not crash when migrating empty classes

[RobinMalfait]

This PR fixes a crash while using npx @tailwindcss/upgrade when migrating classes with no body inside of an @layer utilities.

When running npx @tailwindcss/upgrade, one thing we do is migrate the CSS from:

@layer utilities {
  .foo {
    color: red;
  }
}

To:

@utility foo {
  color: red;
}

We already have some logic that leaves non-classe (IDs, attribute selectors, ...) alone. But if we are migrating a class that has no body, then we will migrate that as well:

@layer utilities {
  .empty {
  }
}

Is turned into:

@utility empty {
}

But later in the migration process this will result in an error because a @utility has to have at least some nodes.

Ideally, you don't even have CSS that has empty rules since it doesn't have any effect in the browser (except of making your CSS file bigger), but it could be that you don't have control over this file, so a fix is still valid.

This PR solves that by leaving those rules alone, and keep them in an @layer utilities.

Fixes: #20204

Test plan

1. Added a dedicated test for this usecase

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the change is a narrow, well-tested guard that keeps empty rules out of the migration path without touching any non-empty rule handling.

The fix is minimal and targeted: a new isEmpty predicate gates the class-collection walk, the cleanup condition is tightened to preserve empty rules in the defaults clone, and a dedicated test covers all four empty-rule variants (truly empty, comment-above, comment-inside, comment-below). No existing tests regress; the snapshot format change is purely cosmetic.

No files require special attention.

_{Reviews (2): Last reviewed commit: "update changelog" | Re-trigger Greptile}

[coderabbitai]

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• CHANGELOG.md

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Walkthrough

This PR enhances the migrateAtLayerUtilities codemod: it adds an isEmpty() helper to skip rules/containers with no nodes or only comment nodes when collecting utility class names, removes contiguous trailing comment nodes alongside removed rules, refines cleanup to treat default and non-default @layer utilities clones differently (removing empty wrappers/rules only from non-default clones), and normalizes raws.before on the default clone. Test snapshots and the local migrate helper were updated to reflect the normalized output formatting.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Do not crash when migrating empty classes' clearly and concisely summarizes the main change: preventing crashes when handling empty class rules during migration.
Description check	✅ Passed	The description is well-detailed and directly related to the changeset, explaining the bug, the problem scenario, the solution, and linking to the relevant issue.
Linked Issues check	✅ Passed	The PR directly addresses issue #20204 by preventing empty class rules from being migrated into empty @utility blocks, implementing the required fix to handle empty rules without crashing.
Out of Scope Changes check	✅ Passed	All changes are focused on the specific issue: adding an isEmpty helper, refining cleanup logic for empty nodes, and adding a test case for empty classes, with no unrelated modifications.

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