Allow includePath for annotation namespace optional

The second parameter of AnnotationRegistry::registerAutoloadNamespace() is optional. Respect this by not enforcing it in application.ini. 

This entry is still mandatory:
`resources.doctrine.orm.entityManagers.default.metadataDrivers.annotationRegistry.annotationNamespaces.0.namespace`

This entry is now optional:
`resources.doctrine.orm.entityManagers.default.metadataDrivers.annotationRegistry.annotationNamespaces.0.includePath`

https://github.com/doctrine/annotations/blob/master/lib/Doctrine/Common/Annotations/AnnotationRegistry.php#L70