For recent saves that don't have an available thumbnail, we should display something.

Steps to test:

1. Set browser.newtabpage.activity-stream.discoverystream.recentSaves.enabled to true
2. Log into Pocket.
3. Save any page to Pocket that does not have a thumbnail, like this one: https://searchfox.org/mozilla-central/source/browser/components/pocket/content/panels/js/components/Home/Home.jsx
4. Open a newtab.
5. Scroll down to the recent saves section.

Expected: You should see the newly saved article with a generic thumbnail, and not a blank white thumbnail.

You can also try a few other pages that don't have thumbnails, I find them pretty easily, but searchfox is a good place.

Each page you save should have a varied, but consistent thumbnail. Duplicates are possible though.

https://hg.mozilla.org/mozilla-central/rev/8f5109ad1e7f