Correctly handle unset/empty items for CORS configurations in response headers policies

[ankon]

Fixes #30440

Description

This should fix #30440, by serializing empty/unset items for these elements to Items:[], Quantity:0.

I was thinking of also making the elements completely optional, but that might be too big a change?

Relations

Closes #30440

Output from Acceptance Testing

% make testacc TESTS=TestAccXXX PKG=ec2

...

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[ankon]

Marking as draft: It looks like the API explicitly rejects empty lists in at least some of these, and that there are special values (*, ALL) that need to be used. Going to iterate a bit here, and see whether that is something that then would be better as "fix by improved documentation", or whether TF should translate these.