Bugfix: Mage_Adminhtml_Helper_Dashboard_Data::countStores() on null

[sreichel]

Part of #4340.

Fatal error:  Uncaught Error: Call to a member function getItems() on null in /var/www/html/app/code/core/Mage/Adminhtml/Helper/Dashboard/Data.php:50
Stack trace:
#0 /var/www/html/test.php(68): Mage_Adminhtml_Helper_Dashboard_Data->countStores()
#1 /var/www/html/test.php(79): test4340()

Steps to reproduce (*)

$test = Mage::helper('adminhtml/dashboard_data');
var_dump($test->countStores());