通知学员下载伴奏，曲目地址加默认值，兼容android问题

mec-im/src/main/java/com/ym/pojo/MusicScoreMessage.java

@@ -14,13 +14,13 @@ public class MusicScoreMessage {
     private String subjectName;
 
     /**  */
-    private String mp3Url;
+    private String mp3Url = "";
 
     /** 速度 */
     private Integer speed;
 
     /**  */
-    private String xmlUrl;
+    private String xmlUrl = "";
 
     /**  */
     private String examSongName;
@@ -35,7 +35,7 @@ public class MusicScoreMessage {
     private String type;
 
     /**  */
-    private String url;
+    private String url = "";
 
     public Integer getId() {
         return id;