DERBY-2002: Case expression allows NULL in all parts of <result>

Require at least one expression with a known type (that is, not NULL or
a ? parameter) in the THEN and ELSE clauses of a CASE expression.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1599142 13f79535-47bb-0310-9956-ffa450edef68