Don't crash on unbound variables

default.nix

@@ -38,7 +38,7 @@
     file="$FRAJTANO_DIR/cookie"
     (umask 077; [ -f "$file" ] || ${pkgs.coreutils}/bin/head -c 128 /dev/urandom | ${pkgs.coreutils}/bin/base64 -w0 > "$file")
     export RELEASE_COOKIE=$(${pkgs.coreutils}/bin/cat "$file")
-    [ -n $RELEASE_NODE ] || export RELEASE_NODE="frajtano-$(${pkgs.coreutils}/bin/whoami)@$(${pkgs.coreutils}/bin/cat /etc/hostname)"
+    [ -n ''${RELEASE_NODE:-} ] || export RELEASE_NODE="frajtano-$(${pkgs.coreutils}/bin/whoami)@$(${pkgs.coreutils}/bin/cat /etc/hostname)"
     run() {
       exec ${lib.getExe pkg} "$@"
     }