stdenv: fix propagatedUserEnvPkgs when __structuredAttrs is true

This is a follow-up of #388908.

Previously, $out/nix-support/propagated-user-env-packages was not
created when __structuredAttrs is true, the first element of
propagatedUserEnvPkgs is null and the length of propagatedUserEnvPkgs
is at least 2.

Fixes #388829

pkgs/stdenv/generic/builder.sh

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

pkgs/stdenv/generic/setup.sh

···
 
     # Propagate user-env packages into the output with binaries, TODO?
 
-    if [ -n "${propagatedUserEnvPkgs:-}" ]; then
+    if [ -n "${propagatedUserEnvPkgs[*]:-}" ]; then
         mkdir -p "${!outputBin}/nix-support"
         printWords "${propagatedUserEnvPkgs[@]}" > "${!outputBin}/nix-support/propagated-user-env-packages"
     fi