Add more nullability annotations to module/spring-boot-actuator-autoconfigure

See gh-46587

Author: mhalbritter

module/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/server/ManagementContextFactory.java

@@ -67,6 +67,7 @@ public ConfigurableApplicationContext createManagementContext(ApplicationContext
 		}
 		ConfigurableApplicationContext managementContext = ApplicationContextFactory.DEFAULT
 			.create(this.webApplicationType);
+		Assert.state(managementContext != null, "'managementContext' must not be null");
 		managementContext.setEnvironment(childEnvironment);
 		managementContext.setParent(parentContext);
 		return managementContext;