Minor Description
The Scala streams DSL for left and outer joins should use Options instead of possibly-null parameters to the joiner functions. As currently written, implementers of joiners for Kafka Streams must account for the right side of a left join (or either side of an outer join) being null, which is not idiomatic Scala.
Note to reviewer: I would like to contribute this change myself. What is the policy on breaking API changes? Would it be acceptable to change the left/outer join APIs to accept options? Or would I need to implement parallel versions (naming suggestions if so?).