Until 2.2.1, custom tags must have an attribute, or else the following error is thrown on configuration parsing: The child node "attributes" at path "ezpublish.ezrichtext.custom_tags.separator" must be configured.

For simple tags, it is realistic that there are no attributes at all (like an <hr />). Not defining the attributes key at all shouldn't generate any error, both on configuration parsing and rendering (the attributes twig variable should be an empty array).