Fix a bug in log forwarder's s3 handler to properly handle SNS records

[ge0Aja]

What does this PR do?

Fix a bug that was introduced during a recent refactoring to handle SNS records on receiving S3 events.

Motivation

Testing Guidelines

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• This PR contains breaking changes that are documented in the description
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