core: VdsNetworkInterface to support empty properties map

Instead of implicitly assuming that custom properties are either null
or a meaningful collection, a method is added to check whether
meaningful properties exist, and the DAO is updated as well to not
trash the DB with empty collections.

Change-Id: Ie1cad7133800b4b8c2b7eb7e8be0be80a1e5cb9b
Signed-off-by: Lior Vernia <lvernia@redhat.com>