Skip initialization of a `NullBean`

[sbrannen]

In AbstractAutowireCapableBeanFactory, initializeBean() currently attempts to initialize a NullBean.

However, invokeInitMethods() (which is invoked by initializeBean()) skips processing of a NullBean with a bean.getClass() != NullBean.class check, which is logical since a NullBean will never contain init-methods.

In practice, initialization and post-processing of a NullBean should not result in any change to the NullBean, thus we should simply skip initialization of a NullBean altogether.