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