ui: lvm-thin: don't add nodeselector/enable multiple times in edit window

ui: lvm-thin: don't add nodeselector/enable multiple times in edit window

Seemingly, an empty array is special for Ext JS, and if we push into
it, we overwrite the class definition of that property.

This does not happen when something is already in the array, so the
other edit windows work as expected

To fix it here, remove the empty array entirely, since we already
create an empty one when we need it

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>