nushell: fix small eval issue, core make vm creation simple

modules/core.nix

···
   config = lib.mkIf (config.my.username != "root") {
     users.users.${config.my.username} = {
       home = "/home/${config.my.username}";
+      initialPassword = "";
       isNormalUser = true;
       extraGroups = [
         "networkmanager"

modules/dev/shell/nushell.nix

···
           show_banner: false,
         }
 
-        if ("~/.profile.nu" | path exists) {
-          source-env "~/.profile.nu"
-        }
+        source-env (if ("~/.profile.nu" | path exists) {"~/.profile.nu"} else null)
 
         ${lib.meta.getExe pkgs.pokego} -l french
       '';