Polish resource handling tests
Prepend leading slash in ResourceUrlProvider

The getForRequestUrl method of ResourceUrlProvider uses the
HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE attribute to
determine the relevant portion of the resource URL path.

However there are cases when that attribute may not have a leading
(e.g. when the current URL was matched to a prefix-based pattern
and hence extracted via PathMatcher#extractPathWithinPattern), which
interferes with the matching of resource URL paths to patterns.

This change ensures a leading slash is present

Issue: SPR-12281
Proper JCache metadata caching on non cache methods

Prior to this commit, the cache operation metadata cache was not
updated for a method not using the JCache annotations. This means
that every execution of said method was going through the process
of identifying if it was using the cache or not.

This commit adds a default placeholder identifying the absence of
metadata; this allows to flag such method as not having any metadata
at all.

Issue: SPR-12337
Fix issue with async return value type determination

Before this change, the type of asynchronously produced return values
(e.g. Callable, DeferredResult, ListenableFuture) could not be
properly determined with an actual resulting value of null. Or even
with an actual value returned, the generic type could not be properly
determined. This change fixes both of those issues.

Issue: SPR-12287
Polish
Add check for ResponseEntity<void> with null return value

Issue: SPR-12287