do not serialize isEmpty

Author: or-else

tinodesdk/src/main/java/co/tinode/tinodesdk/model/MsgGetMeta.java

@@ -200,6 +200,7 @@ private void buildWhat() {
         what = sb.toString();
     }
 
+    @JsonIgnore
     public boolean isEmpty() {
         return mSet == 0;
     }