CMS 6 REGRESSION - Can't move files/folders up to the root level

[GuySartorelli]

Dragging files and folders over the back button in the asset admin () will move those files up to the parent folder.

In CMS 5 this also works with moving them to the root folder (id 0), but in CMS 6 trying to move them to the root folder results in an error.
This is likely a result of the refactor that replaced the GraphQL queries with regular POSTs to controller routes.

Make sure to add a behat test for this when fixing it.