check if resolutions are set before attempt to sort
check if resolutions are set before attempt to sort

--HG--
branch : 0.8.x

Anderson0026/mapproxy,camptocamp/mapproxy,vrsource/mapproxy,geoadmin/mapproxy,drnextgis/mapproxy,camptocamp/mapproxy,procrastinatio/mapproxy,mapproxy/mapproxy,kaiCu/mapproxy,faegi/mapproxy,drnextgis/mapproxy,faegi/mapproxy,mapproxy/mapproxy,kaiCu/mapproxy,procrastinatio/mapproxy,geoadmin/mapproxy,olt/mapproxy,vrsource/mapproxy,olt/mapproxy,Anderson0026/mapproxy