webadmin: class cast exception in AdElementListModel

A regression has been introduced by fc169177820737e984c65d930056be7e2e85d289.

If there is no domain configured the search query used to return ArrayList but
now it returns a Collections.emptyList() while the AdElementListModel tries to
cast it to ArrayList which fails.

Fixed by casting it to more general Collection.

Change-Id: Ib3e3d4b18227027ca31aaab80f403c1fd813fee6
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>