MediaWiki core currently requires the ability to construct empty Content objects for any content model, via ContentHandler::makeEmptyContent. To support this even for kinds of entities that cannot be completely empty (like Properties and Lexemes), we need to allow EntityContent to be created with no Entity object.

Note that EntityContent can already exist with no Entity attached - in case the EntityContent is a redirect. This task asks for the ability to create EntityContent objects that are not redirects and do not contain an Entity.