drainer: process empty string from tidb in update/delete statement on oracle

[cartersz]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   if binlog hava empty string value in column, drainer will generate " col='' " in where clause in sql.
   but oracle treate empty string as null, oracle need "col is null" to match column.

2. What did you expect to see?
   if col value is empty string, drainer will generate "col is null" in update/delete statement.

3. What did you see instead?

4. Please provide the relate downstream type and version of drainer.
   we need bug fix release 6.1