Performance optimization by replacing is_null with === null 

According to http://framework.zend.com/issues/browse/ZF-5413 the usage of is_null is slow.
So I replaced every occurrences of is_null or !is_null with === null or !== null

Unit test are running "normal".

First time for me, so I do accept your Contributor License Agreement.

PS: This pull request contains all commits. Sorry.