web: use '_unset' default value for proxy config method

This special value is needed to make sure registered default value are taken in
account.

mercurial/hgweb/hgwebdir_mod.py

@@ -491,7 +491,7 b' class hgwebdir(object):'
             else:
                 yield config('web', 'motd', '')
 
-        def config(section, name, default=None, untrusted=True):
+        def config(section, name, default=uimod._unset, untrusted=True):
             return self.ui.config(section, name, default, untrusted)
 
         self.updatereqenv(req.env)