log4net AdoNetAppender插入“Null”字符串而不是数据库中的Null

I have a log4net logging on my .net 3.5 application. Logging is inserting into database. One issue I am having is that for the database which i doesn't set data it inserts "NULL" instead of database null. My config is I have a log4net logging on my .net 3.5 applica