tests: fix python env leak test

Author: sandydoo

tests/git-hooks-no-python-leak/devenv.nix

@@ -3,7 +3,7 @@
   git-hooks.hooks.nixfmt-rfc-style.enable = true;
 
   enterTest = ''
-    if [ -n "$PYTHONPATH" ]; then
+    if [ -n "''${PYTHONPATH:-}" ]; then
       echo "PYTHONPATH is non-empty: $PYTHONPATH" >&2
       echo "The pre-commit package is leaking its dependencies into the environment." >&2
       exit 1