nixos/cgit: make checkExportOkFiles not nullable again

When I introduced the checkExportOkFiles option in
a722a999d1 I missed `(cfg.settings ? strict-export)`
in the assertions. This was fixed in bf9ccd1a68
but that also made the option nullable (which is confusing for a required option)
and introduced a small type error. This now makes the option not nullable again
and also fixes the type error.

Fixes #478320.

nixos/modules/services/networking/cgit.nix

@@ -217,8 +217,7 @@ in
                   When enabled you must also configure `strict-export = "git-daemon-export-ok"`
                   in `settings` to make cgit check for the same files.
                 '';
-                type = lib.types.nullOr lib.types.bool;
-                default = null;
+                type = lib.types.bool;
               };
             };
           }
@@ -234,11 +233,6 @@ in
           assertion = !cfg.enable || (cfg.scanPath == null) != (cfg.repos == { });
           message = "Misconfigured services.cgit.${vhost}: Exactly one of scanPath or repos must be set.";
         }
-        {
-          assertion =
-            cfg.enable -> cfg.gitHttpBackend.enable -> cfg.gitHttpBackend.checkExportOkFiles != null;
-          message = "Misconfigured services.cgit.${vhost}: When gitHttpBackend.enable is true then gitHttpBackend.checkExportOkFiles must be set, see the documentation for the option for further information.";
-        }
         {
           assertion =
             cfg.enable