In this case it is safer to compare with Time field. What if it is a problem with just this song?

internal/handlers/handlers.go

@@ -135,7 +135,7 @@ func (h *RadioHandlers) LastSong(w http.ResponseWriter, r *http.Request) {
 		h.logErr.Println("cannot retrieve last songs:", err)
 	}
 
-	if song.Artist == "" {
+	if song.Time == "" {
 		w.WriteHeader(http.StatusNotFound)
 		return
 	}