stdenv: handle null replaceStdenv as identity (#481399)

    stdenv =

      assert vanillaPackages.stdenv.hostPlatform == localSystem;

      assert vanillaPackages.stdenv.targetPlatform == localSystem;

      config.replaceStdenv { pkgs = vanillaPackages; };

      let

        fn = config.replaceStdenv or null;

      in

      if fn == null then vanillaPackages.stdenv else fn { pkgs = vanillaPackages; };

  })

]