Add a missing null check to KinesisHeaderable

[Otanikotani]

What does this PR do?

Adds a missing null check into KinesisHeaderable. This is needed to properly parse records without _datadog attribute

Motivation

We got some NPEs in our logs and figured that is causing the issue.

Testing Guidelines

Added a new unit test

Additional Notes

Types of changes

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

Checklist

• [x] This PR's description is comprehensive
• [] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[Otanikotani]

Sorry I missed that null check in the first PR.