EntityMalformedException when previewing unsaved entities

Problem/Motivation

When previewing changes to an entity that makes use of breadcrumbs, the rendered entity is an unsaved entity object different from the original one and doesn't have an ID, so calling toUrl() on it throws an EntityMalformedException: The "%s" entity cannot have a URI as it does not have an ID.

Proposed resolution

For the case where $entity->isNew(), build the breadcrumb by only using the homepage element and the current page element, if any of those are set. We could skip the breadcrumb entirely, but keeping it with a partly-valid value is better for design comparison purposes.