Breaking change in News 15.1.0

[Grotax]

Brief description of the issue.

With news 15.1.0 we have a change in the feed attribute "folderId", if the feed is in the root folder the id will be "null" instead of 0.

See https://github.com/nextcloud/news/issues/939

How to reproduce the bug?

1. Install 15.1.0-rc2
2. sync
3. ...

What was the expected result?

Accept both 0 and null as a value at least for a while to allow for a smooth transition.