fix CatalogLinkPluginBase if used with stride

[haricot]

related jrief/djangocms-cascade#376
typeError: super(type, obj): obj must be an instance or subtype of type

[2020-04-14 06:45:54,625 log] ERROR: Internal Server Error: /en/admin/cms/page/plugin/cascade_clipboard_plugin/import-plugins/
Traceback (most recent call last):
  File "/.cache/pypoetry/virtualenvs/my-shop-guMgNwqw-py3.8/lib/python3.8/site-packages/django/template/base.py", line 829, in _resolve_lookup
    current = current[bit]
TypeError: 'BootstrapImagePluginElement' object is not subscriptable

...
  File "/.cache/pypoetry/virtualenvs/my-shop-guMgNwqw-py3.8/lib/python3.8/site-packages/django/template/base.py", line 837, in _resolve_lookup
    current = getattr(current, bit)
  File "/.cache/pypoetry/virtualenvs/my-shop-guMgNwqw-py3.8/src/djangocms-cascade/cmsplugin_cascade/link/plugin_base.py", line 68, in link
    return self.plugin_class.get_link(self)
  File "/.cache/pypoetry/virtualenvs/my-shop-guMgNwqw-py3.8/src/django-shop/shop/cascade/plugin_base.py", line 109, in get_link
    return super().get_link(obj) or link_type
TypeError: super(type, obj): obj must be an instance or subtype of type