Fix SslStress docker build (#72940)

Author: antonfirsov

src/libraries/System.Net.Security/tests/StressTests/SslStress/run-docker-compose.sh

@@ -71,7 +71,7 @@ if [[ "$buildcurrentlibraries" -eq 1 ]]; then
 fi
 
 build_args=""
-if [[ -z "$imagename" ]]; then
+if [[ -n "$imagename" ]]; then
     build_args=" --build-arg SDK_BASE_IMAGE=$imagename"
 fi