fix #1762 (#1959)

dbsync/src/main/java/com/taobao/tddl/dbsync/binlog/JsonConversion.java

@@ -317,7 +317,7 @@ public class JsonConversion {
                     buf.append("true");
                     break;
                 case LITERAL_NULL:
-                    buf.append("NULL");
+                    buf.append("null");
                     break;
                 case OPAQUE:
                     String text = null;