Fix registry center 'Unknown column a.undefined' error and Oracle DDL null/not null logic inversion

[safishhh]

注册中心-文档 查询时报 Unknown column 'a.undefined'

Oracle DDL 时字段 null/not null 反了

[aiwenmo]

Hi. Thank you very much for your code contribution.
I think the fix for the Oracle non-null error can be merged. Regarding the issue in the Document Center, we need to find the root cause instead of handling it by modifying the xml.