do not display attributions if empty string
do not display attributions if empty string
openlayers/openlayers,stweil/openlayers,ahocevar/openlayers,bjornharrtell/ol3,ahocevar/openlayers,stweil/openlayers,bjornharrtell/ol3,openlayers/openlayers,openlayers/openlayers,ahocevar/openlayers,stweil/openlayers,bjornharrtell/ol3