`Feed.validate()`: don't crash on `.thumbnail_url` not set (`required=False` doesn't work…).
`Feed.validate()`: don't crash on `.thumbnail_url` not set (`required=False` doesn't work…).

WillianPaiva/1flow,WillianPaiva/1flow,1flow/1flow,1flow/1flow,WillianPaiva/1flow,WillianPaiva/1flow,1flow/1flow,WillianPaiva/1flow,1flow/1flow,1flow/1flow