DERBY-6567: Incorrect nullability for CASE expression with parameter

Make sure the result of a CASE expression is nullable if (and only if)
at least one of the result expressions is nullable.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1593495 13f79535-47bb-0310-9956-ffa450edef68