Cannot add emtpy feed

[sukiletxe]

Add-on version
Add-on version: 3.4

Describe the bug
Currently I can't add empty feeds (use case: comment feeds where: there are no comments; I've commented; but the comment is pending moderation (so the feed is still empty).

To Reproduce
Steps to reproduce the behavior:

1. Go to https://ctrl.blog and look for posts with no comments (there should be plenty)
2. Click on post comment.
3. Click on feed, then try subscribing.
4. Nothing happens.

Expected behavior
The add-on should have added the empty feed to the livemarks.

[evilpie]

We usually consider empty feeds to be some kind of parsing problem and thus don't allow adding them.

[sukiletxe]

Woops, forgot to reply to this. If I understand the RSS and ATOM specs correctly, it seems that RSS requires only title, link and description, and ATOM only id, title and updated. Granted, I don't know how valid feeds are in the web, but feeds without items are technically allowed.

[nt1m]

We should probably allow subscribing to empty feeds.