Since SPR-17034, the core container now behaves a bit differently when
dealing with `null` beans.

Given a `null` `HandlerMapping` bean named "resourceHandlerMapping":

* `context.getBean("resourceHandlerMapping", HandlerMapping.class)`
still returns a `NullBean`
* `ListableBeanFactory.getBeansOfType` will return a Map of all existing
beans, not including the `null` ones as values of the map.

Closes gh-13760