fix(macos): don't crash on undefined variable

Author: dvarrazzo

scripts/build/build_libpq.sh

@@ -62,7 +62,7 @@ case "$ID" in
         # openssl no matter what so remove it. Since homebrew's curl depends on
         # it, force use of system curl.
         brew uninstall --force --ignore-dependencies openssl gettext curl
-        if [ -z "$MACOSX_ARCHITECTURE" ]; then
+        if [ -z "${MACOSX_ARCHITECTURE:-}" ]; then
             MACOSX_ARCHITECTURE="$(uname -m)"
         fi
         # Set the deployment target to be <= to that of the oldest supported Python version.