builder.sh: check if variable is non null

This is not the "correct" way to check if a variable is non null in
bash. There is already an instance of the "right" way to do it in
setup.sh. Bash is "generous" enough to accept the original input though.
I couldn't find the relevant shellcheck.

pkgs/stdenv/generic/builder.sh

@@ -18,6 +18,6 @@ mkdir "$out"
 # Allow the user to install stdenv using nix-env and get the packages
 # in stdenv.
 mkdir $out/nix-support
-if [ "$propagatedUserEnvPkgs" ]; then
+if [ -n "${propagatedUserEnvPkgs:-}" ]; then
     printf '%s ' $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages
 fi