Fixes strange error in tests

On a virgin Neo4j server, the first test will always fail because the first created node obviously gets id 0. This ```int(0)``` is compared to the actual primary key, which is (on a newly created node) ```NULL```. But ```NULL == int(0)```. With this in mind, the EntityManager will not assign any primary key to the very first created node on a fresh up server. This makes tests fail on travis CI.

A simple equation comparison ```NULL !== int(0)``` fixes it. And makes tests work.

Fixes #71 and makes #70 work: https://travis-ci.org/frne/Neo4j-PHP-OGM/builds/24647938