Fix WebMvc auto-conf tests after Framework change

This commit adapts to a recent Spring Framework change (a40d25a) that
turns no-op Spring MVC beans (infrastructure components that, given
their configuration, won't contribute anything to the application) into
`NullBean` instances.

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

@@ -191,9 +191,7 @@ public class WebMvcAutoConfigurationTests {
 	public void resourceHandlerMappingDisabled() {
 		this.contextRunner.withPropertyValues("spring.resources.add-mappings:false")
 				.run((context) -> {
-					Map<String, List<Resource>> locations = getResourceMappingLocations(
-							context);
-					assertThat(locations).isEmpty();
+					assertThat(context.getBean("resourceHandlerMapping")).isEqualTo(null);
 				});
 	}