avro

AVRO-3644: handle java.util.Optional as a nullable value (#1915)

Why:
Javas Optional type can not be used in any meaningful way with the
reflection based schema generation. It is for example not possible
to write a generic custom encoding.

How does it help with resolving the issue:
Optional can be used to make fields null safe in their usage.

Side effects:
I think that this change should not have any side effects, because
Optional fields could not have been used in the past.