fix: add KMS policy statement only if key given

[kayman-mk]

Description

As we assumed that the KMS key is always present (why isn't it?), the policy is created without any resource in case we don't have any key. This PR introduces a separate policy created only if the key is present.

Closes #1257

[github-actions]

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[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ COPYPASTE	jscpd	yes		no	1.4s
✅ REPOSITORY	checkov	yes		no	18.1s
✅ REPOSITORY	dustilock	yes		no	0.28s
✅ REPOSITORY	gitleaks	yes		no	1.0s
✅ REPOSITORY	git_diff	yes		no	0.01s
✅ REPOSITORY	grype	yes		no	10.92s
✅ REPOSITORY	secretlint	yes		no	0.68s
✅ REPOSITORY	syft	yes		no	1.13s
✅ REPOSITORY	trivy-sbom	yes		no	0.08s
✅ REPOSITORY	trufflehog	yes		no	2.68s
✅ SPELL	cspell	3		0	2.02s
✅ TERRAFORM	terraform-fmt	2	0	0	0.1s

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