fix(events): omit KmsKeyIdentifier from Archive when no KMS key is configured

[Zelys-DFKH]

Issue # (if applicable)

Closes #37847.

Credit to @pahud, who did the full investigation here — the root cause and fix direction are his. This is the implementation.

Reason for this change

When Archive is created without a kmsKey prop, the L2 construct synthesizes KmsKeyIdentifier: "" into the CloudFormation template. An empty string is not a valid value for KmsKeyIdentifier: when no CMK is configured, the property should be absent so CloudFormation falls back to the AWS-managed default. In some regions, the empty string causes a GeneralServiceException on deployment.

The EventBus L2 already handles this correctly: it passes props?.kmsKey?.keyArn with no fallback. Archive was inconsistent. The comment claiming the empty string was "necessary to enforce an update that removes any previously used CMK" is also wrong: removing a CMK means omitting the property, not supplying an empty string.

Description of changes

• archive.ts: removed the || '' fallback from kmsKeyIdentifier, so the property is undefined (and therefore absent from the synthesized template) when no key is provided. Removed the two misleading comment lines above the CfnArchive constructor call.
• archive.test.ts: added Match to the import. Renamed the no-key test to better describe what it verifies. Updated the assertion from KmsKeyIdentifier: '' to KmsKeyIdentifier: Match.absent().

Describe any new or updated permissions being added

None.

Description of how you validated changes

All five archive unit tests pass:

• creates an archive for an EventBus
• creates an archive for an EventBus with a pattern including a detailType property
• should have defined defaultChild
• Archive with a customer managed key on an event bus
• Archive without a KMS key omits KmsKeyIdentifier from the template

The CMK test still asserts KmsKeyIdentifier is present and matches the key ARN. The CMK path is unaffected.

No existing snapshot tests reference KmsKeyIdentifier: "", so no snapshot updates are required.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[Zelys-DFKH]

Two CI jobs show failures that are unrelated to the code change:

• pr-triage-manager — labeling bot error, not code-related
• cli-changes — the step-security/changed-files action errored with "Unable to locate commit sha" because it runs on the upstream's shallow pull_request_target checkout and can't resolve fork commit SHAs across the fork boundary. This PR doesn't touch cx-api/lib/features.ts or any CLI files; no CLI integ tests are needed.

Neither is required for merge. Happy to investigate further if helpful.

[Zelys-DFKH]

Exemption Request

The integ test requirement makes total sense for changes that affect real AWS resource behavior. This one is different. The bug is that KmsKeyIdentifier: "" was being written into the CloudFormation template when no KMS key is configured. An empty string isn't a valid value; the property should simply be absent.

The fix is a one-line || '' removal in archive.ts. The behavior is fully observable at synthesis time and the updated unit tests cover it directly: KmsKeyIdentifier: Match.absent() on the no-key path, and the CMK path still asserts the key ARN is present. A deployment-level integ test would spin up an EventBridge Archive with and without a KMS key: substantial AWS overhead to verify a template property being absent vs. empty string.

If the team thinks a deployment test is warranted here, I'll add one. Just checking first.

[Zelys-DFKH]

CI is red on one integration test snapshot. Dropping the empty KmsKeyIdentifier changes the synthesized template, so integ.archive comes back CHANGED.

I have not run cdk-integ with --dry-run or edited the snapshot by hand, since CONTRIBUTING asks contributors not to. Could a maintainer run the integration test so it gets regenerated from a real deployment?