fix(s3-deployment): skip replacement pass when no source markers

[garysassano]

Issue # (if applicable)

Closes #37234.

Reason for this change

BucketDeployment still walks extracted files for marker replacement even when a source has no markers. That adds unnecessary work for deployments that have nothing to substitute.

Description of changes

• Add an early return in the Python aws-s3-deployment handler after extraction when the current source has no markers.
• Keep the existing replacement behavior unchanged for sources that do contain markers.
• Add a regression test to verify the handler does not call replace_markers() when the marker map is empty.

Describe any new or updated permissions being added

No new or updated IAM permissions are required.

Description of how you validated changes

• Added a unit test for the empty-marker fast path.
• Ran bash packages/@aws-cdk/custom-resource-handlers/test/aws-s3-deployment/bucket-deployment-handler/test.sh.

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]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	24 ran	24 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	24 ran	24 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates