Reenable file proxy when renaming between mount points

When moving a folder into another mount point, $renamedFiles is empty
because that goes over a different mechanism.

In such case, this fix makes sure that the file proxy is reenable to
avoid breaking the subsequent files that are being moved.
Merge pull request #11167 from owncloud/env-movetomountreenableproxy

Reenable file proxy when renaming between mount points