we may not have windows enabled, just don't log anything

git-svn-id: https://xpra.org/svn/Xpra/trunk@21159 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/server/shadow/shadow_server_base.py

@@ -96,7 +96,11 @@ def do_print_screen_info(self, display, w, h):
             log.info(" on display '%s' of size %ix%i", display, w, h)
         else:
             log.info(" on display of size %ix%i", w, h)
-        l = len(self._id_to_window)
+        try:
+            l = len(self._id_to_window)
+        except AttributeError as e:
+            log("no screen info: %s", e)
+            return
         if l>1:
             log.info(" with %i monitors:", l)
             for window in self._id_to_window.values():