KSQL UDF and UDAFs can have non-optional return schemas

[big-andy-coates]

The fields in the row schema in KSQL as supposed to always be optional. This is work @rodesai did previously. This is important because at the moment if a UDF throws an exception while processing a field KSQL outputs the row with a null value in the field. So fields must be nullable.

However, while introducing KsqlSchema recently I noted there are some points in the code base where this is not being adhered to: UDF and UDAF return values!

Obviously, in time we'll likely want to support NOT NULL column defs. But that requires some thought! Until that happens, we should be consistent and always have nullable fields.