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.

Fixes https://github.com/owncloud/core/issues/11162

Note: I did not remove the warnings, they are logged as debug anyway.

Please review @schiesbn @LukasReschke @th3fallen 

To test, use the steps from #11162 but have at least TWO files inside the folder. One of them will be broken before this fix.