HPCC-9879 Fix cores with unicode find strength functions on null strings

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4787 from ghalliday/issue9879

HPCC-9879 Fix cores with unicode find strength functions on null strings 

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>