Route key '_only_fragment' is changed to '_no_path'

Some special routes in core, like <fragment> use a route option which results in just displaying the fragment.
In #2351015: URL generation does not bubble cache contexts this was expanded to also show the query:

Before

@@ -394,7 +394,7 @@ system.theme_settings_theme:
 '<none>':
   path: ''
   options:
    _only_fragment: TRUE
   requirements:
     _access: 'TRUE'

After

 '<none>':
   path: ''
   options:
    _no_path: TRUE
   requirements:
     _access: 'TRUE'