fix(amplify-alpha): removing customResponseHeaders property does not remove the headers from the app

[georeeve]

Issue # (if applicable)

Closes #31783.

Reason for this change

After setting the customResponseHeaders property to some value in the App construct, removing it again does not remove the headers in the Amplify console.

Description of changes

I have set the customHeaders property to a default value of an empty string. This sets the relevant CloudFormation property when customResponseHeaders is not provided, therefore overriding any previous values with an empty string. I took a look at a similar issue of removing basic auth from an app and saw we did something similar there - #15243.

Description of how you validated changes

Updated the existing tests with the new default value.

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]

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: ed717cb
• Result: SUCCEEDED
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[georeeve]

Exemption Request - I have updated the existing unit test, and integration test snapshots with the new default values. I'm not really sure what I could put in a new integration test, as it'd test the same values as the existing integration tests.

[aws-cdk-automation]

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