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

Most of the code uses `=== null`. 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. There is no need to invoke a method for a simple `=== null` check. This is not noticeable for some higher methods, but some Utility ones like in Hash or CakeTime class might be used quite a few times per request, so there it might make a difference in the long run, as well.

Also corrected the last wrong assert method param orders.

EDIT:
I did not touch the comparisons in https://github.com/cakephp/cakephp/blob/master/lib/Cake/Model/CakeSchema.php#L554 by the way.
As there it is used in some shortcut way to check equality.
I could resolve it as
`if ($value === null && $correspondingValue !== null || $value !== null && $correspondingValue === null) {`
if desired.