Add `unless` option to Cacheable

I have some cache conditions where the value can only be cached if the value is not null.
Apparently there is now way to do it in version 4, since the result of the method call can not be accessed inside the condition closure.

Before, I had the following condition: "#result != null", now I would need to rewrite it to something like
`{
result != null
}`

Start by locating the Cacheable entry point and reading how version 4 evaluates its condition closure. Compare that behavior with the reported former "#result != null" condition. Done means an unless option can express the non-null result case and the behavior is covered by tests.

Written by the indexing model from the issue text.