Defined in github.com/aws/aws-sdk-go/service/iam/api.go
The date and time, in ISO 8601 date-time format (http://www.iso.org/iso/iso8601), when the access key was most recently used. This field is null when:
The user does not have an access key. An access key exists but has never been used, at least not since IAM started
tracking this information on April 22nd, 2015.
There is no sign-in data associated with the user