fix issue with Avro record default values in Kafka Connect converter

[jurgispods]

Issue #, if available: #307

Description of changes: See issue for description of the problem. This PR adds a check for JsonProperties.NULL_VALUE in addition to null to fix this problem.

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[jurgispods]

Did anyone have a chance to look at this?

[YangSan0622]

Can you add unit test for this feature and ensure the existing integration test is passing after this change?

[jurgispods]

@YangSan0622 I copied the same unit test used in confluentinc/schema-registry#1928 - mvn test runs successfully.

[YangSan0622]

Have you run the integration tests in https://github.com/awslabs/aws-glue-schema-registry/tree/master/integration-tests?

[jurgispods]

@YangSan0622 Running mvn test from the integration-test directory runs through, but run-local-tests.sh fails with a few "Access denied" errors. Is there any documentation on how to run the tests? It seems the tests need an AWS account with certain permissions. If that is the case, it would be very helpful if you could run the tests in your environment.

[jurgispods]

Hi, any update on this? I will gladly run the integration tests, given I know how to do so and I have permission to access the corresponding AWS resources. Otherwise, I am dependent on the help of a maintainer.