HPCC-9817 Fix issues comparing against a NULL unicode string

Strange as it may seem, the ICU compare functions check for NULL
pointers as parameters, and sometimes abort early giving the wrong result.
See http://sourceforge.net/mailarchive/message.php?msg_id=29406824 for
a similar issue.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>