addCondition can create OR expr. + NULL fixes

Patches from 4.2.5:

https://github.com/atk4/atk4/commit/be7be48f7c754258df09f279729fefb2b25a95e9
https://github.com/atk4/atk4/commit/ee6d8074921e112ef4d3f391ab567cc4b4daac7b