HPCC-8733 cmake STREQUAL should be protected from empty strings

IF (${XYZ} STREQUAL "XYZ") will cause cmake configuration error if XYZ is blank.

Fixes HPCC-8733

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #3934 from GordonSmith/HPCC-8733

HPCC-8733 cmake STREQUAL should be protected from empty strings

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>