Fix timezone controller bad return (#15191)

was returning null when preferences.timezone was not set (not sure how to get to that case)

app/Http/Controllers/Ajax/TimezoneController.php

@@ -46,6 +46,6 @@ class TimezoneController extends Controller
             return $request->timezone;
         }
 
-        return session('preferences.timezone');
+        return session('preferences.timezone', '');
     }
 }