Fix: handle missing argument in build-windows.sh

Fix unbound variable error when script is run without arguments.
Use parameter expansion ${1:-} instead of $1 to safely check for
--installer flag when set -u is enabled.

docker-build/build-windows.sh

@@ -5,7 +5,7 @@ set -euo pipefail
 BUILD_INSTALLER=false
 
 # Check for --installer flag
-if [[ "$1" == "--installer" ]]; then
+if [[ "${1:-}" == "--installer" ]]; then
     BUILD_INSTALLER=true
     echo "[*] Installer build requested."
 fi