Fixed inconsistent which cause error

If $this->entityChangeSets[$oid] can be empty as we can see from next IF statement but PreUpdateEventArgs constructor accepts only array it will cause error like this

```
Argument 3 passed to PreUpdateEventArgs::__construct() must be of the type array, null given, called in vendor/doctrine/orm/lib/Doctrine/ORM/UnitOfWork.php on line 1028 and defined
```

It fixes http://www.doctrine-project.org/jira/browse/DDC-2726 and #126