Upgrade problem

Hi

When upgrading to HEAD I got the error

Warning: array_key_exists() expects parameter 2 to be array, null given
in /home/daniel/www/DCMS/vendor/twig/twig/lib/Twig/Environment.php line
975

Caused by adding globals in the initRuntime method in my extensions.

This patch makes addGlobal handle the case where Envieonment::$globals is null.