dbus: fix nil notifications (#2180)

When mangling notifications via naughty.config.notify_callback it is
advised (1) to return nil to reject a notification. The underlaying dbus
library tries to access a field of that notification table and fails.

(1) https://awesomewm.org/doc/api/libraries/naughty.html

lib/naughty/core.lua

@@ -420,6 +420,12 @@ function naughty.getById(id)
     end
 end
 
+--- Increase notification ID by one
+function naughty.get_next_notification_id()
+  counter = counter + 1
+  return counter
+end
+
 --- Install expiration timer for notification object.
 -- @tparam notification notification Notification object.
 -- @tparam number timeout Time in seconds to be set as expiration timeout.

lib/naughty/dbus.lua

@@ -191,9 +191,11 @@ capi.dbus.connect_signal("org.freedesktop.Notifications",
                 end
                 args.freedesktop_hints = hints
                 notification = naughty.notify(args)
-                return "u", notification.id
+                if notification ~= nil then
+                    return "u", notification.id
+                end
             end
-            return "u", "0"
+            return "u", naughty.get_next_notification_id()
         elseif data.member == "CloseNotification" then
             local obj = naughty.getById(appname)
             if obj then