#146 Artist List sort by-material is not listing Clay Artists

`<staging:8080>/artist/sorted/by-material` Artist sort by-material does not list Clay Artists unless they have another associated material. e.g. * If an artist has `Clay` and `Metal` as materials it shows up under `Clay` list. * If an artist only has `Clay` as material then it doesn't show under the `Clay` list. I would like... * for materials to list all available in their respective categories.