Backport of Allow AWS secret engine to send empty policy document into release/1.14.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #23470 to be assessed for backporting due to the inclusion of the label backport/1.14.x.

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The person who merged in the original PR is:
@Monkeychip
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/vault/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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This fixes a regression caused by a PR here that fixed a larger issue.

AWS is a unique case where we can't default the json form field to an empty object. A user needs to return nothing if no policy_document is added.

With the fix:

Screen.Recording.2023-10-03.at.11.29.27.AM.mov

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Overview of commits

• 9774cb0

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Name	Status	Preview	Comments	Updated (UTC)
vault	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 4, 2023 3:07pm

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CI Results:
All Go tests succeeded! ✅