item_enclosure_url is None, leading to a TypeError with mimetypes.guess_type(url)

[gabn88]

See https://github.com/Fantomas42/django-blog-zinnia/blob/b4949304b104a8e1a7a7a0773cbfd024313c3a15/zinnia/feeds.py#L133

I have not been able to trigger it myself, but I have seen it happen frequently in our error logs, especially with crawlers/security tests.