Query.NullHandlingOnOrder does not work on SQLserver

When testing with SQLServer, we found that `Query.NullHandlingOnOrder` contsructs queries in flowable that cannot be executed by MS SQLserver. I am not sure if I should post this as a bug report to mybatis instead?

The corresponding stacktrace can be found here: https://gist.github.com/mlippens/3689a43228e3f86422b160e324efcbcc

As for the version of mssql, we use the docker linux image https://hub.docker.com/r/microsoft/mssql-server-linux/, but we could also reproduce it with an actual install of SQLServer.

从 Query.NullHandlingOnOrder 开始，针对 SQL Server 重现报告中的查询，并使用链接 Gist 中的 stacktrace 作为初始诊断参考。为排序中的 null 处理生成的查询能够在 SQL Server 上成功执行，即视为完成。

由索引模型根据 Issue 内容生成。