fix: null site variable (from https://forum.getfuelcms.com/discussion…

…/3162/fuel-sitevariables-model-php)

fuel/modules/fuel/models/Fuel_sitevariables_model.php

@@ -101,7 +101,7 @@ public function retrieve_all()
 	public function retrieve_one($name = null)
 	{
 		$vars = $this->find_one_array(array('active' => 'yes', 'name' => $name));
-		return $vars['value'];
+		return !empty($vars) ? $vars['value'] : NULL;
 	}
 
 	// --------------------------------------------------------------------