naughty: Set a default reason in `:destroy()`.

Fix #2713

lib/naughty/core.lua

@@ -360,6 +360,8 @@ end
 
 -- Remove the notification from the internal list(s)
 local function cleanup(self, reason)
+    assert(reason, "Use n:destroy() instead of emitting the signal directly")
+
     if properties.suspended then
         for k, v in pairs(naughty.notifications.suspended) do
             if v == self then

lib/naughty/notification.lua

@@ -249,6 +249,8 @@ function notification:destroy(reason, keep_visible)
           return false
     end
 
+    reason = reason or cst.notification_closed_reason.dismissed_by_user
+
     self:emit_signal("destroyed", reason, keep_visible)
 
     self._private.is_destroyed = true