data-source/aws_rds_cluster: Prevent panic when CloudWatch logs are enabled

[bflad]

Fixes #4922

While I can add an acceptance test to cover this specific case, it does bring up an interesting point. This data source failure would only occur in a certain resource situation and was not caught by existing acceptance testing. Much like we can run into trouble if all acceptance testing doesn't run the ImportStateVerify check for all iterations of a resource, we can run into similar situations with data sources as well. Thoughts appreciated on future thinking about how to handle this better.

Changes proposed in this pull request:

• Add enabled_cloudwatch_logs_exports attribute to aws_rds_cluster data source to match resource

Output from acceptance testing:

1 test passed (all tests)
=== RUN   TestAccDataSourceAwsRdsCluster_basic
--- PASS: TestAccDataSourceAwsRdsCluster_basic (103.40s)

[bflad]

This has been released in version 1.25.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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