[Bugfix] OSM Stamen Toner is added to GetPrint even if corresponding layer is not defined
[Bugfix] OSM Stamen Toner is added to GetPrint even if corresponding layer is not defined

Fixes #874

laurentj/lizmap-web-client,3liz/lizmap-web-client,nboisteault/lizmap-web-client,3liz/lizmap-web-client,nboisteault/lizmap-web-client,rldhont/lizmap-web-client,laurentj/lizmap-web-client,3liz/lizmap-web-client,rldhont/lizmap-web-client,nboisteault/lizmap-web-client,laurentj/lizmap-web-client,3liz/lizmap-web-client,rldhont/lizmap-web-client,nboisteault/lizmap-web-client,rldhont/lizmap-web-client,laurentj/lizmap-web-client