Allow for falsey, non-null identifiers in persistNew()
Fix getPartialReference() test to not use invalid identifier per mapping

This leaves a todo item for registerManaged(). As-is, this test failed because registering a document with an identifier not valid for its mapping would lead to an inconsistency in the UnitOfWork maps. The CmsUser document's identifier uses IdType, so we should use a MongoId (or compatible string) here.
Use base test's DocumentManager in IdTest