fix(terraform): fix a TF plan issue with CKV_AWS_274

[gruebel]

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Description

• in some cases policy_arn can be non existent, when a TF plan file is created, it will be populated after the apply

Fixes #3744

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
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• I have added tests that prove my feature, policy, or fix is effective and works
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