fix: #3275276 Removing a relationship that is used by a filter can prevent access to editing a view

Bundle::init() resolved its entity type through the handler's relationship, which throws when the relationship has been removed from the display, or when none has been chosen yet on a view whose base table has no entity type. Take the entity type from the table the filter is on instead, and let HandlerBase::getEntityType() fall back to the base table when the relationship is missing, so DisplayPluginBase::validate() can report it.

Closes #3275276