GET mailrepository/{repository}/mails/{id} should not return null fields when not asked in request

[chibenwa]

See https://issues.apache.org/jira/browse/JAMES-2438

Webadmin endpoint to retrieve an email should include default fields and additionalFields required in input request but should not include null fields for additionalFields actually not computed on server.

You can have a look to https://stackoverflow.com/questions/13764280/how-do-i-exclude-fields-with-jackson-not-using-annotations for dynamically ignoring fields.