Null values are not cached

Is it intended that `null` value is not cached?
I think it has to be configurable, because `null` is valid value for me, or am I missing something?

Example:

```kotlin
@Cacheable("test")
fun getCompanyForUser(id: Long): String? {
// implementation
}
```

Not every user has to have company, but when the method returns `null`, it's not cached, but called every time. I didn't even find this behaviour documented.

I am using `io.micronaut.cache:micronaut-cache-caffeine:2.0.0.M2`.

Start with the Kotlin @Cacheable example and the reported micronaut-cache-caffeine 2.0.0.M2 behavior, then trace how null results are handled. Done means the null-caching behavior can be configured and the behavior is documented, with coverage for the example scenario.

Written by the indexing model from the issue text.