[3006.x] Fix x509_v2 local signing with signing policy

[lkubb]

What does this PR do?

Ensure the state module does not crash when locally signing with a signing policy that defines signing_private_key.

What issues does this PR fix or reference?

Fixes: https://github.com/saltstack/salt/issues/66414

Previous Behavior

The certificate is created, but subsequent runs crash.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

Commits signed with GPG?

Yes