Register default resource path using a Resource

Update `WebMvcAutoConfiguration` so that the default "/" resource path
is registered directly as a `ServletContextResource`.

Closes gh-24745

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageHandlerMapping.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,6 @@ package org.springframework.boot.autoconfigure.web.servlet;
 
 import java.util.Collections;
 import java.util.List;
-import java.util.Optional;
 
 import javax.servlet.http.HttpServletRequest;
 
@@ -49,9 +48,9 @@ final class WelcomePageHandlerMapping extends AbstractUrlHandlerMapping {
 	private static final List<MediaType> MEDIA_TYPES_ALL = Collections.singletonList(MediaType.ALL);
 
 	WelcomePageHandlerMapping(TemplateAvailabilityProviders templateAvailabilityProviders,
-			ApplicationContext applicationContext, Optional<Resource> welcomePage, String staticPathPattern) {
-		if (welcomePage.isPresent() && "/**".equals(staticPathPattern)) {
-			logger.info("Adding welcome page: " + welcomePage.get());
+			ApplicationContext applicationContext, Resource welcomePage, String staticPathPattern) {
+		if (welcomePage != null && "/**".equals(staticPathPattern)) {
+			logger.info("Adding welcome page: " + welcomePage);
 			setRootViewName("forward:index.html");
 		}
 		else if (welcomeTemplateExists(templateAvailabilityProviders, applicationContext)) {

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -814,7 +814,7 @@ class WebMvcAutoConfigurationTests {
 	protected Map<String, List<Resource>> getResourceMappingLocations(ApplicationContext context) {
 		Object bean = context.getBean("resourceHandlerMapping");
 		if (bean instanceof HandlerMapping) {
-			return getMappingLocations(context.getBean("resourceHandlerMapping", HandlerMapping.class));
+			return getMappingLocations((HandlerMapping) bean);
 		}
 		assertThat(bean.toString()).isEqualTo("null");
 		return Collections.emptyMap();

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

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,6 @@ package org.springframework.boot.autoconfigure.web.servlet;
 
 import java.util.Collections;
 import java.util.Map;
-import java.util.Optional;
 
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -162,8 +161,7 @@ class WelcomePageHandlerMappingTests {
 			return new WelcomePageHandlerMapping(
 					templateAvailabilityProviders
 							.getIfAvailable(() -> new TemplateAvailabilityProviders(applicationContext)),
-					applicationContext, Optional.ofNullable(staticIndexPage.getIfAvailable()), staticPathPattern);
-
+					applicationContext, staticIndexPage.getIfAvailable(), staticPathPattern);
 		}
 
 	}