Mob 1950

@nandy-andy @aquilax @rogatty @RafLeszczynski This is for the API to return null whenever it should display the default marker (no image or image not ready) + status so the client side can play with it. DO NOT MERGE YET, let's wait for Zach's decision if to keep the default marker.
Move it before ` utils.convertMarkersNamesToUrls() `, it will save us some processing time.