Prior to this commit, AbstractAutowireCapableBeanFactory's
initializeBean() method always attempted to initialize a NullBean.
However, invokeInitMethods() (which is invoked by initializeBean())
skips processing of a NullBean, 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.
This commit therefore skips initialization of a NullBean altogether.
Closes gh-35165