This commit handles "empty" cases for `ResponseEntity` controller
handler return types when wrapped with a `java.util.Optional` in Spring
MVC or a single `Publisher` like `Mono`.

Given the following example for Spring MVC:

```
@GetMapping("/user")
public Optional<ResponseEntity<User>> fetchUser() {
	Optional<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

If the resulting `Optional` is empty, Spring MVC will infer a
`ResponseEntity` with an empty body and a 404 HTTP response status.

The same reasoning is applied to Spring WebFlux with Publisher types:

```
@GetMapping("/user")
public Mono<ResponseEntity<User>> fetchUser() {
	Mono<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

This feature is only valid for `HttpEntity` return types and does not
apply to `@ResponseBody` controller handlers.

Issue: SPR-13281

Issue: SPR-17178

Issue: SPR-17180
Issue: SPR-17182

Includes Groovy 2.5.2, Undertow 2.0.12, Hibernate Validator 6.0.12.

Issue: SPR-17129

Rename "Builder" sub-section to "Configuration" and move it in the
beginning before all others since it explains how to create a client
in the first place.

Update content on Reactor Netty connector based on the API in 0.8 and
specifically address Reactor Netty resources and lifecycle.

Issue: SPR-16963

Issue: SPR-16963

Issue: SPR-17120

Prior to this commit, ScriptUtils supported MySQL-style escapes ('\\')
when splitting a script into statements; however, MySQL-style escapes
were not supported when determining if a given script contained a
specified statement delimiter. This caused executeSqlScript() to
erroneously fallback to a newline as the statement separator in such
cases.

This commit fixes this issue by implementing the same check for
MySQL-style escapes in containsSqlScriptDelimiters() that was already
present in splitSqlScript().

Issue: SPR-17120

Its parameter DefaultCacheConfig is just package-visible...

Issue: SPR-16946

Includes revised InstantiationStrategy nullability for args array.

Issue: SPR-17171

Issue: SPR-17174

This commit enables the `-use` javadoc flag so that class usage pages
are included in the aggregated JavaDoc that is published to
https://docs.spring.io/spring-framework/docs/.

Issue: SPR-17173

First step (as proof of concept) toward addressing SPR-17174.

Issue: SPR-17174

When expanding and strictly encoding URI variables, there is no need to
quote `/` and `$` which will be encoded anyway.

Issue: SPR-17168

Issue:SPR-17142

javax.xml.stream.Location is already there as an import there is no need
to use the FQCN in the handleDtd method.

No that JCacheCache extends AbstractValueAdaptingCache instead of
directly implementing Cache an import statement can be used in favor
of using the FQCN for the field and constructor arguments.

Issue: SPR-17169

JmsListenerAnnotationBeanPostProcessor also ignores JmsListenerContainerFactory and JmsListenerEndpointRegistry, avoiding unnecessary annotation introspection on framework classes.

Issue: SPR-17166
Issue: SPR-16933

Issue: SPR-17167

Issue: SPR-17166

ScheduledAnnotationBeanPostProcessor also ignores TaskScheduler and ScheduledExecutorService, avoiding unnecessary annotation introspection on framework classes.

Issue: SPR-17166
Issue: SPR-16933

Issue: SPR-15535

Issue: SPR-17154

Issue: SPR-16946

Issue: SPR-17034

Issue: SPR-17163