it seems gtk.Screen.get_monitor_plug_name can return None, so replace…

… it with an empty string to avoid error in network encoder

git-svn-id: https://xpra.org/svn/Xpra/trunk@1409 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/client.py

@@ -713,7 +713,7 @@ def get_screen_sizes(self):
             monitors = []
             while j<screen.get_n_monitors():
                 geom = screen.get_monitor_geometry(j)
-                monitor = (screen.get_monitor_plug_name(j), geom.x, geom.y, geom.width, geom.height,
+                monitor = (screen.get_monitor_plug_name(j) or "", geom.x, geom.y, geom.width, geom.height,
                             screen.get_monitor_width_mm(j), screen.get_monitor_height_mm(j))
                 monitors.append(monitor)
                 j += 1