Issue #, if available: n/a

Description of changes:

This change copies across the getMessageAttributes implementation from the SQS sdk. Return an empty list instead of null if no message attributes are available.

https://github.com/aws/aws-sdk-java/blob/3c367441d1c5d2f21e10d54cb4f2b308a8b00c59/aws-java-sdk-sqs/src/main/java/com/amazonaws/services/sqs/model/Message.java#L684-L686

Target (OCI, Managed Runtime, both):

both?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.