Null point Exception during loading the checksum (#1788)

If you connect to the mysql with version 5.1, the result of command 'select @master_binlog_checksum' is an array with only one Null element .

parse/src/main/java/com/alibaba/otter/canal/parse/inbound/mysql/MysqlConnection.java

@@ -521,7 +521,7 @@ public class MysqlConnection implements ErosaConnection {
         try {
             rs = query("select @@global.binlog_checksum");
             List<String> columnValues = rs.getFieldValues();
-            if (columnValues != null && columnValues.size() >= 1 && columnValues.get(0).toUpperCase().equals("CRC32")) {
+            if (columnValues != null && columnValues.size() >= 1 && columnValues.get(0) != null && columnValues.get(0).toUpperCase().equals("CRC32")) {
                 binlogChecksum = LogEvent.BINLOG_CHECKSUM_ALG_CRC32;
             } else {
                 binlogChecksum = LogEvent.BINLOG_CHECKSUM_ALG_OFF;