etebase-server: fix the eval on null cfg.unixSocket

Without the change the test eval fails as:

    $ nix build --no-link -f. etebase-server.tests
       error: cannot coerce null to a string: null

nixos/modules/services/misc/etebase-server.nix

@@ -177,6 +177,7 @@ in
 
     systemd.tmpfiles.rules = [
       "d '${cfg.dataDir}' - ${cfg.user} ${config.users.users.${cfg.user}.group} - -"
+    ] ++ lib.optionals (cfg.unixSocket != null) [
       "d '${builtins.dirOf cfg.unixSocket}' - ${cfg.user} ${config.users.users.${cfg.user}.group} - -"
     ];