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://bugs.icu-project.org/trac/ticket/9396

(Based on 4.0.0, but pull request against 4.0.2).

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>