Allow for undefined OldImage in Dynamo stream events

[benmartin-coforma]

Description

We create Kafka messages from AWS event streams, which can be triggered by activity in DynamoDB. Usually, the event contains both a "before" and "after" of the object - but when the event is an insert, there is no "before". That is, event.Records[n].dynamodb.OldImage will be undefined. Unless we account for this, it will cause an error when we attempt to unmarshall it.

This is a low-impact bug; subsequent updates to that record will go through just fine, because they will have an OldImage. This PR just cleans up the initial edge case.

Related ticket(s)

CMDCT-3431

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How to test

🤷‍♂️ I guess we just merge it, and then watch the logs from our postKafkaData lambda?

Notes

n/a

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Pre-review checklist

• I have performed a self-review of my code
• I have added thorough tests, if necessary
• [ ] I have updated relevant documentation, if necessary

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Pre-merge checklist

Review

• [ ] Design: This work has been reviewed and approved by design, if necessary
• [ ] Product: This work has been reviewed and approved by product owner, if necessary

Security

If either of the following are true, notify the team's ISSO (Information System Security Officer).

• [ ] These changes are significant enough to require an update to the SIA.
• [ ] These changes are significant enough to require a penetration test.

[codeclimate]

Code Climate has analyzed commit 01bb681 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (90% is the threshold).

This pull request will bring the total coverage in the repository to 79.3%.

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