Add Support for JS Resolver DDB BatchGetItem Projections

The [docs](https://docs.aws.amazon.com/appsync/latest/devguide/js-aws-appsync-resolver-reference-dynamodb-batch-get-item.html) indicate support, but when I add a `projection` to my `BatchGetItem`, AppSync returns a `NullPointerException` with the message: `Cannot invoke "Object.toString()" because the return value of "java.util.Map.get(Object)" is null`

Start with the linked AWS AppSync BatchGetItem documentation and reproduce the reported JavaScript resolver request with a projection. Confirm whether the documented projection shape consistently triggers the NullPointerException; done means the request succeeds with the expected projected attributes or the discrepancy is fully documented.

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