How do I get a default value to appear when using the map method? What used to work on v2.x doesnt on 3.2

Example:

In my Model I have:

    public function getListOfSites(){

        if($sites = DataObject::get('Site'))
            return $sites->map('ID', 'Title', 'All');

    }

Then in Page.php I want to add the sites to the settings tab like so

$fields->addFieldToTab('Root.Settings', new DropdownField('Site', 'Select a site that will see this article', Site::getListOfSites()));

Where "All" is was what appeared as the default value in a drop down menu in v2.x but no longer works... What else can I do?

The parameters $emptyString and $sort of DataObjectSet->map() got removed. Please use DropdownField->setEmptyString() instead.

$site = new DropdownField('Site', 'Select a site that will see this article', array());
$site->setEmptyString('All');
$fields->addFieldToTab('Root.Settings', $site);

Thanks! Spent so much time trying to figure out the correct way of doing it. Your solution worked great