Session::getAll() may return null

I experienced an error when i cleared the website cookie: in this case `SilverStripe\Control\Session::getAll()` is returning `null` and not an empty array (which would be a nice suggestion to ss-repo), thus leading to a foreach-loop error.

code/Collector/SilverStripeCollector.php

@@ -98,6 +98,11 @@ public static function getCookieData()
     public static function getSessionData()
     {
         $data = DebugBar::getRequest()->getSession()->getAll();
+
+        if (empty($data)) {
+            return [];
+        }
+
         $filtered = [];
 
         // Filter not useful data