This API call:
# profile: https://www.pixiv.net/fanbox/creator/40684196
curl -H "Origin: https://fanbox.cc" "https://api.fanbox.cc/creator.get?userId=40684196"
returns `{ "body": nil }` when the artist is deleted. We didn't expect `body` to be nil.
Also fix it so that `profile_url` returns the `https://www.pixiv.net/fanbox/creator/40684196`
URL if we can't get the `https://<username>.fanbox.cc` URL, usually because the API call failed
because the artist is deleted.