Empty fallback image generates a wrong path

Problem/Motivation

The Empty fallback, which generates the img tag with src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7", was generating an HTTP request, because $base_url is appended to the src since the img was generated using theme_image function. This causes wrong HTTP requests, when there should be no HTTP requests.

Proposed resolution

Change the image formatter to image_srcset, which already has a "hack" for this particular situation.

Remaining tasks

Needs to be reviewed, tested and patched. (I'm using it on a production environment, with the "src" fallback enabled)