iOS: Silent Notifications

[TCB13]

Ability to send "silent" notifications to iOS devices.

https://stackoverflow.com/a/8949642/560745

At AppleNotification.php:61 we should check if $this->alert is empty:

 if (is_array($this->alert)) {
            if (!empty($alert)) {
                $alert   = array_intersect_key($this->alert, array_fill_keys($this->supportedAlertProperties, 0));
                $payload += ['alert' => $alert];
            }
        } else if (is_scalar($this->alert)) {