[Transfer] Server and Connector optional fields should not be returned in response

[jflim]

https://docs.aws.amazon.com/boto3/latest/reference/services/transfer/client/describe_server.html
https://docs.aws.amazon.com/transfer/latest/APIReference/API_DescribedServer.html

In practice, it appears that optional values that are not set shouldn't be included in the responses.
Fields such as LoggingRole and others should be omitted from responses

https://github.com/getmoto/moto/blob/master/moto/transfer/types.py

[jflim]

Working on fixing the issue