Skip initialization of a NullBean

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

spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java

@@ -1798,6 +1798,11 @@ public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFac
 	 */
 	@SuppressWarnings("deprecation")
 	protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
+		// Skip initialization of a NullBean
+		if (bean.getClass() == NullBean.class) {
+			return bean;
+		}
+
 		invokeAwareMethods(beanName, bean);
 
 		Object wrappedBean = bean;