fix: MalformedPolicyDocument error without kms or ssm arns

[brycelowe]

Description

I'd like the option to remove access to KMS and SSM permissions on my IRSA roles while still providing the ability to use this module with the default encryption key provided by AWS. When I attempt to provide an empty list, the IAM policy is invalid because a resource definition is required.

Error: updating IAM Policy (arn:aws:iam:::policy/role-External_Secrets_Policy-20190815225516998100000001): MalformedPolicyDocument: Policy statement must contain resources.
	status code: 400, request id: <id>

Motivation and Context

Most of the secrets in my environment have been created with the default encryption key, so they don't need any special access to KMS or SSM. When attempting to remove this permission I ran into an error applying the configuration because the policy document was malformed.

Breaking Changes

No, this is not a breaking change as the existing default remains intact.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request