Return an empty list instead of None from OopsWindow.get_selected()

- because of https://retrace.fedoraproject.org/faf/reports/57507/

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Require correct version of libreport

- because of https://retrace.fedoraproject.org/faf/reports/54692/

Signed-off-by: Jakub Filak <jfilak@redhat.com>