rm empty-src placeholder img tag on Click to add more! invite.

The "Marketplace light" representation on the landing page is a virtual Favorite that is an invitation to click to add more.

That virtual favorite had an empty-src image tag, causing a broken image under Firefox e.g.  This changeset drops that src-less image.  It can of course be re-added when there's an image to display there.

Before:

![broken_image_under_firefox_before](https://cloud.githubusercontent.com/assets/952283/4526971/fd9abe30-4d63-11e4-8c43-b6ba7eb431bb.png)


After:

![no_image_at_all_after](https://cloud.githubusercontent.com/assets/952283/4526973/01605b1a-4d64-11e4-939c-2eaaaeca8e5a.png)