DERBY-6569: NULLIF may return incorrect results if first operand calls non-deterministic function

Make sure the left operand of the NULLIF expression is only evaluated
once, so that it does not matter if it is non-deterministic.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1625520 13f79535-47bb-0310-9956-ffa450edef68