fix how we detect an unset variable

Author: vtbassmatt

src/ci/scripts/setup-environment.sh

@@ -11,7 +11,7 @@ source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 # Since matrix variables are readonly in Azure Pipelines, we take
 # INITIAL_RUST_CONFIGURE_ARGS and establish RUST_CONFIGURE_ARGS
 # which downstream steps can alter
-if [[ -n "${INITIAL_RUST_CONFIGURE_ARGS}" ]]; then
+if [[ -v INITIAL_RUST_CONFIGURE_ARGS ]]; then
     ciCommandSetEnv RUST_CONFIGURE_ARGS "${INITIAL_RUST_CONFIGURE_ARGS}"
 fi