Email update fail deployment

[flochaz]

Updating email make next deployment fail which might be fine because we don't really support email updates but error should be more explicit:

user function response: { “StatusCode”: 200, “ExecutedVersion”: “$LATEST”, “Payload”: “null” }

submit response to cloudformation { “Status”: “FAILED”, “Reason”: “TypeError: Cannot read property ‘IsComplete’ of null\n at isComplete

Source code to be udapted to manage onUpdate case and more obvious failure
https://github.com/awslabs/aws-bootstrap-kit/blob/main/source/aws-bootstrap-kit/lib/validate-email-handler/index.ts#L31