DERBY-6423: The expression syntax in CASE's THEN clause doesn't accept boolean value expression

backported change #r1592465 from trunk to 10.10 branch.

Allow boolean value expressions in CASE and NULLIF expressions.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.10@1630516 13f79535-47bb-0310-9956-ffa450edef68