Check fails if bucket is empty

[charles-dyfis-net]

When a bucket has no initial contents, this resource fails with the error:

jq: error (at <stdin>:1): Cannot iterate over null (null)

This is because in this scenario, aws s3api list-objects --query 'Contents[].{LastModified: LastModified}' returns null, and max_by(.LastModified) is not a valid jq query.

Acceptance Criteria

• [ ] The check for an empty bucket should return an empty list of versions, rather than returning an error.