We do not (currently) consider other ID types as 'end points' justifying
to create an override hierarchy, however if the 'root' ID (i.e. the ID
the user selected as base to create the override) is not an object or
collection, we still want to check all of its dependencies.
This fixes e.g. if a material depends on another Empty object, and user
tries to hierarchy-override that material, its Empty dependency not
being overridden.
ce1d023667