Fix issue 558

Fix issue #558

JMSDiExtraBundle has evolved and issue concerning inexistent parent class has been moved from cache warmers to pre-optimizations pass (see AnnotationLoaderPass from JMS DI Extra Bundle).

Except forcing people to define Admingenerator Bundle after JMSDiExtraBundle in AppKernel, the only solution found was to always load Admingenerator class loader to create empty generated file if required.