Update AWSJavaMailTransport to fall back on no-arg AmazonSESClient ctor

The current implementation of the AWSJavaMailTransport class only works if credentials are explicitly supplied, which means that you can't use fallback mechanisms such as environment variables, IAM Roles for EC2, etc. that are supported by the AmazonSimpleEmailServiceClient.

This patch allows you to not specify credentials, which causes the transport to fallback on the no-arg ctor of the service client.