isl_basic_map_sample: explicitly mark input as empty if no sample was found
isl_basic_map_sample: explicitly mark input as empty if no sample was found

Additionally, return this explicitly empty basic map instead of creating
a new one.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

BenzoSM/isl,simbuerg/isl,Distrotech/isl,Meinersbur/isl,BenzoSM/isl,nicolasvasilache/isl,simbuerg/isl,evaautomation/isl,KangDroidSMProject/ISL,inducer/isl-mirror,nicolasvasilache/isl,BenzoSM/isl,PollyLabs/isl,PollyLabs/isl,simbuerg/isl,inducer/isl-mirror,Distrotech/isl,PollyLabs/isl,evaautomation/isl,PollyLabs/isl,KangDroidSMProject/ISL,tobig/isl,Meinersbur/isl,KangDroidSMProject/ISL,BenzoSM/isl,simbuerg/isl,Meinersbur/isl,Distrotech/isl,BenzoSM/isl,tobig/isl,tobig/isl,KangDroidSMProject/ISL,Distrotech/isl,UBERTC/isl,Distrotech/isl,evaautomation/isl,PollyLabs/isl,nicolasvasilache/isl,evaautomation/isl,nicolasvasilache/isl,tobig/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,inducer/isl-mirror,inducer/isl-mirror,UBERTC/isl,UBERTC/isl,UBERTC/isl,Meinersbur/isl,inducer/isl-mirror,simbuerg/isl