caches: skip cache if user is empty.

rhodecode/authentication/base.py

@@ -694,7 +694,9 b' def authenticate(username, password, env'
         log.debug('AUTH_CACHE_TTL for plugin `%s` active: %s (TTL: %s)',
                   plugin.get_id(), plugin_cache_active, cache_ttl)
 
-        user_id = user.user_id
+        user_id = user.user_id if user else None
+        # don't cache for empty users
+        plugin_cache_active = plugin_cache_active and user_id
         cache_namespace_uid = 'cache_user_auth.{}'.format(user_id)
         region = rc_cache.get_or_create_region('cache_perms', cache_namespace_uid)