@XmlElement(nillable=true) annotation on type properties causes json serialization to include null values

`type` properties are annotated with `@XmlElement(nillable=true)`.

This causes `"type":null` to be output in the json seralization of the corresponding objects in user resources (for Email, Entitlement, Role, Im, Photo, X509Certificate).

I tried with `@XmlElement`, removing `(nillable=true)`, and this seems to produce the expected results and doesn't seem to cause issues on a quick test.

Is `nillable=true` necessary, or can we remove it?

Search the Email, Entitlement, Role, Im, Photo, and X509Certificate resource types for @XmlElement(nillable=true) on their type properties. First reproduce JSON serialization with a null type, then compare behavior after removing nillable=true. Done means null type fields are omitted from user-resource JSON without causing issues in the existing quick-test scenarios.

Written by the indexing model from the issue text.