Change `Optional` deserialization from "absent" value into `null`, from "empty"

[cowtowncoder]

(note: see FasterXML/jackson-modules-java8#251 for background with Jackson 2.x)

So, other "reference type" deserializers handle things so that:

1. explicit null token becomes "empty" reference (like new AtomicReference<>(null) or Optional.empty())
2. absent (missing) value becomes Java null

but currently for Optional (2) is not true: instead, it behaves same as explicit null value from JSON.
This is problematic as it is not possible to distinguish "absent" case from explicit null.

We can change behavior for Jackson 3.0.

[cowtowncoder]

NOTE: will likely revert from 3.0.1, see #5335.