GetCoverArt returns placeholder if id is missing

This mimics Subsonic behaviour, even if it contradicts the API documentation, which states `id` is required

Fixes #1139

server/subsonic/media_retrieval.go

@@ -58,10 +58,7 @@ func (c *MediaRetrievalController) getPlaceHolderAvatar(w http.ResponseWriter, r
 }
 
 func (c *MediaRetrievalController) GetCoverArt(w http.ResponseWriter, r *http.Request) (*responses.Subsonic, error) {
-	id, err := requiredParamString(r, "id")
-	if err != nil {
-		return nil, err
-	}
+	id := utils.ParamStringDefault(r, "id", "non-existent")
 	size := utils.ParamInt(r, "size", 0)
 
 	w.Header().Set("cache-control", "public, max-age=315360000")

server/subsonic/media_retrieval_test.go

@@ -37,11 +37,12 @@ var _ = Describe("MediaRetrievalController", func() {
 			Expect(w.Body.String()).To(Equal(artwork.data))
 		})
 
-		It("should fail if missing id parameter", func() {
+		It("should return placeholder if id parameter is missing (mimicking Subsonic)", func() {
 			r := newGetRequest()
 			_, err := controller.GetCoverArt(w, r)
 
-			Expect(err).To(MatchError("required 'id' parameter is missing"))
+			Expect(err).To(BeNil())
+			Expect(w.Body.String()).To(Equal(artwork.data))
 		})
 
 		It("should fail when the file is not found", func() {