unify null checks - avoid method call in favor of strict check

Most of the code uses ===. Only some places use is_null() - and a few mix those even in the same method.
This unifies it.

As a side note:
It also slightly increases performance. This is not noticeable for some higher methods, but some Utility ones like in Hash class or CakeTime might be used quite a few times per request, so there it might make a difference in the long run, as well.