Simplification of return types. No need to return more mixed than necess...

Follow up on https://github.com/cakephp/cakephp/pull/5084

Ruff ideas on how to make the API simpler.
Also some doc block corrections.

Some tests might not be fine. I can check on that again once we discussed it.
I'm :-1: with this change and the above. `null` represents much better a missing value or "none" an empty string is plain confusing