Update PropertyMapper to better support nullability

Refactor `PropertyMapper` so that it no longer calls adapter or
predicate methods by default when the source value is `null`. This
effectively makes all default calls the same as using
`alwaysWhenNotNull` in the previous generation of the code.

For the limited times when you do need to deal with `null` values, the
new `always()` method can be used.

For example,

	map.from(source::method).to(destination::method);

Will not call `destination.method(...)` if `source.method()` returns
`null`.

Where as:

	map.from(source::method).always().to(destination::method);

Will call  `destination.method(null)` if `source.method()` returns
`null`.

This update provides clearer semantics for the API and allows for better
JSpecify nullability annotations. It has also simplified much of our
existing property mapper code.

Closes gh-47024

Co-authored-by: Moritz Halbritter <[email protected]>

Author: philwebb

core/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebProperties.java

@@ -576,13 +576,10 @@ public void setSMaxAge(@Nullable Duration sMaxAge) {
 					map.from(this::getCachePrivate).whenTrue().toCall(control::cachePrivate);
 					map.from(this::getProxyRevalidate).whenTrue().toCall(control::proxyRevalidate);
 					map.from(this::getStaleWhileRevalidate)
-						.whenNonNull()
 						.to((duration) -> control.staleWhileRevalidate(duration.getSeconds(), TimeUnit.SECONDS));
 					map.from(this::getStaleIfError)
-						.whenNonNull()
 						.to((duration) -> control.staleIfError(duration.getSeconds(), TimeUnit.SECONDS));
 					map.from(this::getSMaxAge)
-						.whenNonNull()
 						.to((duration) -> control.sMaxAge(duration.getSeconds(), TimeUnit.SECONDS));
 					// check if cacheControl remained untouched
 					if (control.getHeaderValue() == null) {