nixos/nginx: fix type of sslTrustedCertificate option

The option was added in 1251b34b5bbcd11a7a2974df7bada5d6d47b985d
with type `types.path` but default `null`, so eval failed with
the default setting. This broke the acme and certmgr tests.

cc: @vincentbernat @fpletz

nixos/modules/services/web-servers/nginx/vhost-options.nix

···
     };
 
     sslTrustedCertificate = mkOption {
-      type = types.path;
+      type = types.nullOr types.path;
       default = null;
       example = "/var/root.cert";
       description = "Path to root SSL certificate for stapling and client certificates.";