Removed magic if from IdentifiedEntity::getId

As per code [comment](https://github.com/Kdyby/Doctrine/commit/9dd60db371d1cc876595b72cf9326720560c388b) and according to clarification in commit [comments](https://github.com/Kdyby/Doctrine/commit/9dd60db371d1cc876595b72cf9326720560c388b#commitcomment-3841973).