Fix build after SPR-17034 in Spring Framework

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

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfigurationTests.java

@@ -138,7 +138,7 @@ public class WebMvcAutoConfigurationTests {
 	@Test
 	public void handlerMappingsCreated() {
 		this.contextRunner.run((context) -> assertThat(context)
-				.getBeans(HandlerMapping.class).hasSize(7));
+				.getBeans(HandlerMapping.class).hasSize(5));
 	}
 
 	@Test