Backport of keyring: Fix a panic when decrypting aead with empty RSA block. into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24442 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

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

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Description

Clusters that have gone through several upgrades have be found to include keyring material which has an empty RSA block.

In more recent versions of Nomad, an empty RSA block is omitted from being written to disk. This results in the panic not being present. Older versions, however, did not have this struct tag meaning we wrote an empty JSON block which is not accounted for in the current version.

Spot check for this locally in the code:

ag "WrappedRSAKey != nil {"

ag "WrappedRSAKey != nil && "
nomad/encrypter.go
480:		if wrappedKey.WrappedRSAKey != nil && len(wrappedKey.WrappedRSAKey.Ciphertext) > 0 {
808:	if kekWrapper.WrappedRSAKey != nil && len(kekWrapper.WrappedRSAKey.Ciphertext) > 0 {

Links

closes #24441
related #24383

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

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Overview of commits

• 4e7496d