fix root node deprecation in symfony-config > 4.1

[roverwolf]

Symfony 4.2 deprecates constructing a TreeBuilder without passing root node information.

This adds a backwards-compatible change to remove the deprecation.

ref https://github.com/symfony/symfony/blob/master/UPGRADE-4.2.md#config

[coveralls]

Coverage remained the same at 96.764% when pulling c268217 on opensalt:symfony-config-deprecation into c0e7c74 on Matthimatiker:master.

[coveralls]

Coverage remained the same at 96.764% when pulling c268217 on opensalt:symfony-config-deprecation into c0e7c74 on Matthimatiker:master.